M6: branch-safe context, summaries and long-term story memory

Aligns the inherited AI-DnD memory and context foundation with the history,
authority and state model M3-M5 established. Long stories now reach the narrator
through a bounded, lineage-safe, inspectable context rather than a growing
transcript.

This commit includes the corrective work that followed the independent review in
planning/reports/M6-IMPLEMENTATION-REPORT.md. The first implementation reported
E03 as passing and it was not; the report records that history rather than
hiding it.

What was already correct, and was kept rather than rebuilt

  Memory lineage. Memories already carried (branch_id, depth) and retrieval
  already filtered through the capped-path clause; the ten-step negative control
  was measured passing against b7005e6 before any change here. M6 adds the
  regression tests that pin it, plus provenance and authority on the result.

Summary lineage — both halves

  A summary is a row carrying the coordinate of the last node it covers, and
  eligibility is the same head-capped lineage clause memories use. That alone
  was not enough: generation was seeded from adventures.story_summary, a
  campaign-global column with no lineage, so after a divergence the summariser
  was handed the abandoned line's prose and asked to update it. The row it
  produced was correctly anchored and therefore looked safe while its sentences
  described a story the reader had left.

  Generation is now seeded from summaries.current — the same question the
  context builder asks — so the input and the output are scoped by one rule.
  adventures.story_summary remains a reader-facing mirror for the Plot panel and
  the export bundle, kept in step when a summary is written and when the head
  moves, and nothing authoritative reads it.

Retrieval redundancy

  With a real embedding model, four near-identical memories crowded out the one
  distinctive clue, which survived only because the default memory_top_k is 5.
  Retrieval now drops a candidate that repeats one already chosen, never across
  authority classes, at a threshold measured against the configured embedding
  model. The clue is retrieved at top_k 5, 4 and 3. Ranking itself is unchanged;
  the further factors CONTEXT-AND-MEMORY §20 contemplates remain unimplemented
  and are recorded as such.

Memory authority, budgeting, observability

  Memory.authority is accepted_story or heuristic, classified by the application
  and marked in the prompt; retrieval never writes state. The reply is reserved
  out of the context budget, and an impossible configuration fails clearly
  instead of overflowing. Each derived pass records ok/idle/failed per campaign,
  served by GET /adventures/{id}/derived and shown in Insights, so the M2
  failure — a dead memory bank with a green suite — is visible if it recurs.
  Provider-wiring tests mock no factory.

Also: two pre-existing test-suite leaks fixed; two fixtures that stored one
vector in every memory now use distinct ones, so lineage assertions stay
readable alongside redundancy suppression.

Planning: CONTEXT-AND-MEMORY, TECHNICAL-DESIGN, DATA-MODEL, V1-ACCEPTANCE-TESTS,
BUILD-MILESTONES, VERSION and planning/README updated to describe what exists,
including that a valid E03 test must regenerate a summary after diverging. The
M5 report was rotated to planning/archive/milestone-reports/. No new ADR — every
choice implements a decision the package had already settled.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PWU4gTfLYY6Qq9U7aa9Qw2

backend/app/context/builder.py

@@ -21,8 +21,9 @@ block and the live sections in `build_context`.
 from dataclasses import dataclass
 
 import tiktoken
+from sqlalchemy.orm import object_session
 
-from .. import models, narrative, worldstate
+from .. import derived, models, narrative, summaries, worldstate
 from . import encoding, history
 
 AUTHORS_NOTE_DEPTH = 3  # actions from the end of history
@@ -63,6 +64,24 @@ MAX_LENGTH_FLOOR_WORDS = 300
 # Built from the table vendored in `encoding.py`, not fetched: the upstream
 # `tiktoken.get_encoding("cl100k_base")` downloads it on first use, and this
 # is called on every turn.
+# M6: added to the configured reply budget when reserving output space. It
+# absorbs the section separators added after budgeting and the drift between
+# this tokenizer and the serving model's. Fixed rather than proportional: what
+# it covers does not grow with the size of the budget.
+OUTPUT_SAFETY_MARGIN = 64
+
+
+class ContextOverflow(RuntimeError):
+    """Raised when protected context alone cannot fit in the token budget.
+
+    Protected means the narrator rules, the campaign canon, the authoritative
+    narrative state, the reader's own input, and the reserve for the reply
+    (`CONTEXT-AND-MEMORY.md` §30). None of those may be dropped to make room for
+    old prose, so when they do not fit there is no prompt to build and saying so
+    is the only honest answer.
+    """
+
+
 def _encoding() -> tiktoken.Encoding:
     return encoding.get_encoding()
 
@@ -187,6 +206,12 @@ def _history_text(action: models.Action) -> str:
     return action.text
 
 
+def _memory_line(memory: dict) -> str:
+    """One retrieved memory, marked with its authority (M6)."""
+    mark = " [inferred]" if memory.get("authority") == "heuristic" else ""
+    return f"-{mark} {memory['text']}"
+
+
 def _canon_section(adventure: models.Adventure) -> str:
     """The campaign's own rules, rendered for the system block.
 
@@ -317,15 +342,33 @@ def build_context(
     # memories change on most turns, and the stat values change on nearly every
     # turn. `world_lore` is added below, because the history window determines
     # which cards trigger and that window is not known yet.
+    # M6: the summary the *current lineage* is entitled to, not whatever was
+    # written last. A summary is derived data anchored to the story it covers,
+    # so an Undo or a divergence makes an old one ineligible rather than
+    # leaking it into a story it does not describe (E03, `app/summaries.py`).
+    db = object_session(adventure)
+    summary_row = summaries.current(db, adventure) if db is not None else None
+    summary_text = summary_row.text.strip() if summary_row is not None else ""
     summary_section = (
-        Section("story_summary", f"Story summary:\n{adventure.story_summary.strip()}")
-        if adventure.story_summary.strip()
+        Section("story_summary", f"Story summary:\n{summary_text}")
+        if summary_text
         else None
     )
     memories_section = None
     if memory_bank and memory_bank.get("used"):
-        lines_text = "\n".join(f"- {m['text']}" for m in memory_bank["used"])
-        memories_section = Section("used_memories", f"Memories:\n{lines_text}")
+        # M6: an inference must not read as a record. A heuristic memory is
+        # marked in the prompt itself, because the narrator decides what to
+        # treat as established from what it is shown, and an unlabelled guess
+        # sitting beside accepted history is how a guess becomes canon
+        # (`CONTEXT-AND-MEMORY.md` §14). Authoritative state changes still come
+        # only from the M5 event path, whatever a memory says.
+        lines_text = "\n".join(_memory_line(m) for m in memory_bank["used"])
+        memories_section = Section(
+            "used_memories",
+            "Memories from earlier in the story. Lines marked [inferred] are "
+            "interpretation, not established fact — do not treat them as "
+            f"settled truth:\n{lines_text}",
+        )
     world_state_section = None
     refusal_note = ""
     # M5: the authoritative narrative state, as the model is shown it. Read from
@@ -370,7 +413,35 @@ def build_context(
         + count_tokens(narrative.extract.EMIT_REMINDER)
         + count_tokens(refusal_note)
     )
-    available = max(256, settings.context_token_budget - reserved)
+
+    # ----- M6: the output reserve, and what happens when it does not fit -----
+    #
+    # `context_token_budget` is the whole window the model is given, so the
+    # narrator's reply has to be subtracted from it before any history is
+    # chosen. Until M6 it was not: the builder spent the entire budget on input
+    # and left the reply to fit in whatever the endpoint had left, which is a
+    # truncated turn on a model whose window is the budget
+    # (`CONTEXT-AND-MEMORY.md` §32, acceptance test F04).
+    #
+    # The margin covers what is added after this arithmetic — the separators
+    # between sections, and the difference between our tokenizer's count and the
+    # serving model's. It is small and fixed rather than proportional, because
+    # what it absorbs does not scale with the budget.
+    output_reserve = max(0, settings.max_output_tokens) + OUTPUT_SAFETY_MARGIN
+    protected = reserved + output_reserve
+    if protected >= settings.context_token_budget:
+        # Failing here is the point. The alternative — carrying on with a token
+        # or two of history — builds a prompt that is known to overflow, and
+        # the reader gets a truncated reply with no explanation. §32: "fail
+        # gracefully if protected context alone is too large."
+        raise ContextOverflow(
+            f"The protected context needs {protected} tokens "
+            f"({reserved} of prompt plus {output_reserve} reserved for the "
+            f"reply) but the context budget is {settings.context_token_budget}. "
+            "Raise the context budget, lower the maximum reply length, or "
+            "shorten the campaign's canon, instructions and persona."
+        )
+    available = settings.context_token_budget - protected
 
     # Only the newest actions can reach the prompt, because the code below
     # either truncates the text to `available` tokens or stops at the budget.
@@ -475,12 +546,28 @@ def build_context(
             {"label": s.label, "text": s.text, "tokens": s.tokens} for s in all_sections
         ],
         "prompt": {"system": system_text, "story": story_text},
+        # M6: the numbers the reader needs to answer "how much did each part
+        # cost, and what was left for the reply?" (F04, F05). `available` is
+        # what the history was actually allowed to spend after everything
+        # protected was subtracted.
         "tokens": {
             "total": count_tokens(system_text) + count_tokens(story_text),
             "budget": settings.context_token_budget,
+            "output_reserve": output_reserve,
+            "protected": reserved,
+            "available_for_history": available,
+            "history_spent": spent,
         },
         "cards": card_records,
         "memories": memory_bank,
+        # M6: which summary was used, and which stretch of story it covers, so
+        # "what history did that summary cover?" is answerable from the record
+        # rather than by guessing (F05, F06).
+        "summary": summaries.provenance(summary_row),
+        # M6: whether background derived work is currently failing for this
+        # campaign. A dead memory bank is visible here rather than only in a log
+        # nobody reads (F08).
+        "derived": derived.report(db, adventure.id) if db is not None else [],
         "history": {
             "included": len(included_actions),
             # The count covers the whole story rather than the window fetched