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/README.md

@@ -3,9 +3,10 @@
 **This file is the index. Start here.**
 
 **Current state:** Phase 0 complete; AI-DnD forked as the production base;
-milestones **M1, M2, M3 and M4 implemented and accepted** (M3 and M4:
-2026-09-03).
-**Next:** **M5 — Genre-Neutral Authoritative Narrative State.** Its brief has not
+milestones **M1 through M6 implemented and accepted** (M3 and M4: 2026-09-03;
+M5: 2026-09-04; M6: 2026-09-06). M5 and M6 were each accepted only after an
+independent review found a real defect and a corrective pass fixed it.
+**Next:** **M7 — First-Class Imported Knowledge Library.** Its brief has not
 been written yet, and writing it is the current action.
 
 **Package version:** see `VERSION.md`, which records what each revision changed
@@ -245,13 +246,14 @@ M5-M11, one at a time               see BUILD-MILESTONES.md
 
 **One milestone at a time. Do not begin a milestone before its brief exists.**
 
-**No M5 brief has been prepared.** Writing one is the current action, informed by
-the M4 report's §U readiness assessment and by the note `BUILD-MILESTONES.md`
-attaches to M5 — in particular that M4 added 55 tests using the inherited
-world-state values as deterministic instrumentation, which M5 must **move rather
-than delete**.
+**No M7 brief has been prepared.** Writing one is the current action, informed by
+the M6 report's §W readiness assessment and by the retrieval debt
+`BUILD-MILESTONES.md` records against M6 — in particular that ranking is
+similarity plus a pin, so imported material will compete for the same memory
+budget as story memory, and that cross-layer duplication
+(`CONTEXT-AND-MEMORY.md` §22) is still open.
 
-**No conditions remain open on M1-M4.** The browser smoke condition that M3 and
+**No conditions remain open on M1-M6.** The browser smoke condition that M3 and
 M4 both carried was satisfied at M4 closeout: a real Firefox exercised the
 rendered DOM for both milestones' controls, 44/44 checks passing. The M3 report's
 §M.2 and the M4 report's §M record the condition as it stood; the M4 report's §W