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

planning/DATA-MODEL.md

@@ -453,6 +453,40 @@ story_thread:
 
 These are narrative continuity tools, not RPG quests.
 
+## 16A. Derived Context Tables (M6, as implemented)
+
+Two tables and one column carry M6's derived context. All three are derived
+data: deleting them changes no accepted history, no authoritative state and no
+head position.
+
+```text
+summaries
+  id, adventure_id
+  text
+  branch_id, depth          the coordinate of the last node covered
+  source_start, source_end  the stretch of story summarized
+  trigger                   "interval" (generated) or "manual" (reader-written)
+  model_name, created_at
+
+derived_status              one row per (adventure, kind)
+  kind                      "memory" | "summary" | "embedding"
+  status                    "ok" (did work) | "idle" (nothing pending) | "failed"
+  detail, failures
+  last_attempt_at, last_success_at
+
+memories.authority          "accepted_story" | "heuristic"
+```
+
+`summaries` mirrors the shape `memories` already had, deliberately: both are
+derived rows anchored to a coordinate on a path, and both are filtered by the
+same lineage clause — and both are also the *input* to the next round of derived
+work, which is why summary generation reads `summaries.current` rather than any
+campaign-global field.
+
+`adventures.story_summary` is retained as the reader's edit surface and the
+export field, mirroring whichever summary is eligible. It carries no lineage of
+its own and nothing authoritative reads it.
+
 ## 17. State Version
 
 The system must reconstruct authoritative state at any retained turn.