M5: genre-neutral authoritative narrative state, with review corrections

Replaces AI-DnD's RPG relative-delta world state with the genre-neutral typed
narrative state of ADR 010: explicit, absolute, allowlisted events proposed by
the model, validated by the application, applied to one authoritative document,
and snapshotted per position so restore stays a row read.

This commit includes the corrective pass that followed the independent review
in planning/reports/M5-IMPLEMENTATION-REPORT.md. The invariant it exists to
hold is:

    visible active transcript position == stored head == authoritative state

Narrator editing (D10, STORY-BRANCH-SEMANTICS §§14-15)

  A narrator edit no longer rewrites a row. It returns to the state before the
  turn, takes the reader's exact text as the accepted narration, re-derives the
  state that text implies, and becomes a new active continuation — while the
  original narration keeps its words, its live flag and its whole future as
  retained history. At the tip the correction is another take; with story below
  it, it forks. No new history machinery: this is the existing fork/take/head
  path with the reader's text in place of a generated reply. The §14A refusal
  is therefore gone for narrator turns, and remains only for player input.

Pre-M5 positions

  Migration 88 backfills the empty narrative document onto every action written
  before M5, and a missing snapshot now restores the empty document instead of
  leaving the previous position's state standing. Restoring to an old Save
  Point no longer leaves a later position's entities and facts on screen.

Narrator context

  Replayed history carries prose only; the machine-readable block is no longer
  reconstructed into past turns, where it contradicted the authoritative state
  in the same prompt. A fact withdrawn by a manual correction is now named as
  no longer true, with the reader's reason, rather than silently dropped.

Also

  - state_changes joins the action-list bulk read, removing one query per row.
  - Extraction takes only the application's own protocol payload: an ordinary
    ```json or ```python block in a story survives, and a mangled proposal
    still does not reach the reader.

Planning: ADR 013 records the authoritative document shape; §§14-15/14A, D10,
C04 and BUILD-MILESTONES are updated to describe what exists. Debt is recorded
against M8 (scenario editor UX) and M9 (export of the audit trail).

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

backend/app/attempts.py

@@ -38,6 +38,7 @@ from sqlalchemy.orm import Session, undefer
 
 from . import models
 from .context import lineage
+from .narrative import model as narrative_model
 
 # The slices of a context snapshot that belong to one attempt rather than to the
 # turn. They are the world-state delta the attempt proposed and what the engine
@@ -45,7 +46,7 @@ from .context import lineage
 # token accounting. Each attempt is its own API call, and a retry is the call
 # most likely to read the prompt back out of cache. Everything else in a snapshot
 # is the prompt, which is assembled once per turn.
-ATTEMPT_KEYS = ("world_state", "raw_output", "usage")
+ATTEMPT_KEYS = ("world_state", "narrative_state", "raw_output", "usage")
 
 
 # ------------------------------------------------------------------ reading
@@ -152,26 +153,67 @@ def preceding(
 # ------------------------------------------------------------------ writing
 
 def restore_state(adventure: models.Adventure, node: models.Action | None) -> None:
-    """Restores the world state that `node` left behind.
+    """Restores the state that `node` left behind.
 
-    A NULL snapshot means leave the live state as it is, never reset it. Rows
-    written before SP4 that the migration could not derive an outcome for carry
-    NULLs, and overwriting a running adventure's state with an empty dict would
-    be worse than doing nothing.
+    This is what makes Undo, Redo, a branch switch and a Save Point restore cost
+    the same at any distance: the destination node carries its own outcome, so
+    arriving is a row read rather than a replay (`TECHNICAL-DESIGN.md` §10.4).
+    M5 changed what is restored, not how — the narrative state document takes
+    the place the RPG world state held, through the same single function.
+
+    The two columns follow **different** rules about a NULL, and the difference
+    is not an oversight.
+
+    For the narrative document, a NULL means *this position established
+    nothing*, and it is restored as the empty document. Leaving the live state
+    alone instead is what the M5 review caught (Finding 3): arriving at a
+    migrated pre-M5 node left a later position's entities, facts and threads
+    standing, so the transcript said depth 2 while the state described depth 6.
+    The invariant this module exists to hold is that the visible position, the
+    head and the authoritative state agree, and "keep whatever was there" cannot
+    hold it. An empty document at an old position is honest — the narrative
+    state system knew nothing then, because it did not exist — where retained
+    state from elsewhere is a claim about a story that had not been told yet.
+
+    Migration backfills those rows explicitly, so this fallback is the belt to
+    that pair of braces: it also covers a node arriving from an older export,
+    which the migration never sees.
+
+    For the legacy RPG world state a NULL still means leave it alone. Those rows
+    predate SP4, nothing consults the values to decide anything, and overwriting
+    a running adventure's numbers with an empty dict would be worse than doing
+    nothing.
     """
     if node is None:
         return
+    adventure.narrative_state = (
+        copy.deepcopy(node.narrative_state_after)
+        if isinstance(node.narrative_state_after, dict)
+        else narrative_model.empty()
+    )
+    # Legacy, and deliberately still restored: a pre-M5 campaign's numbers stay
+    # coherent with the position being read, so an old save is not left showing
+    # a future's values. Nothing consults them to decide anything.
     if isinstance(node.world_state_after, dict):
         adventure.world_state = copy.deepcopy(node.world_state_after)
 
 
 def snapshot_outcome(adventure: models.Adventure, node: models.Action) -> None:
-    """Records on `node` the state of the adventure now that the node has played."""
+    """Records on `node` the state of the adventure now that the node has played.
+
+    Every node, including a player's action that changed nothing. A position
+    without a snapshot is a position the head cannot be restored to, and the
+    head can rest on any node.
+    """
     world = adventure.world_state if isinstance(adventure.world_state, dict) else {}
     # `state_after` held the scripting engine's shared state, which M2 removed.
     # The column stays for schema compatibility and is written empty.
     node.state_after = {}
     node.world_state_after = copy.deepcopy(world)
+    narrative = adventure.narrative_state
+    node.narrative_state_after = copy.deepcopy(
+        narrative if isinstance(narrative, dict) else narrative_model.empty()
+    )
 
 
 def roll_back_before(