fixtures: golden data dir — real records, production read paths

Owner-manufactured teach master (chat_mv27rrra: compaction, subagent-abort,
every teach tool) + practice lifecycle (chat_mv293wv5: the full marker cycle
including the first-ever problem-dismissed), the real graph ledgers, five
jobs across the kind x status matrix, and settings. tests/golden-fixtures
reads all of it through the stores via the LH_DATA_DIR seam and pins the
fixture law: real data parses, flattens to a legal LLM wire, derives the
graph, covers every custom-marker kind, and reads never write.

Fixture law: an intentional format change updates these files in the same
commit; an unintentional one fails the suite.

tests/fixtures/golden-data/README.md

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+# Golden fixtures — real user data, read through production paths
+
+**The law (owner ruling, 2026-10-10):** this directory is a REAL data dir —
+verbatim records written by the real application code, never hand-edited. Any
+intentional on-disk format change updates these fixtures IN THE SAME COMMIT;
+an unintentional change fails `tests/golden-fixtures.test.ts`. When schema
+stamps land (Wave D), the stamping commit updates these files with it.
+
+`tests/golden-fixtures.test.ts` copies this dir to a temp dir, points the
+production stores at it via the `LH_DATA_DIR` seam, and reads EVERYTHING
+through the real read paths — chat records (contract folds + the LLM wire
+flattening), graph ledgers (shape guards + the full derivation), jobs,
+settings. It also pins marker COVERAGE: every custom-marker kind and the
+core tool surface must appear in real data here.
+
+## What's inside
+
+- `chats/chat_mv27rrra_feff23b6.json` — the teach-side master (owner-built,
+  2026-10-10): probe-handoff, plan-block, compaction, subagent-steer,
+  subagent-abort, and every teach tool (quiz, ask_user_question, plan_init,
+  probe, probe_done, recall, explain, research, demo, visual, pdf, find,
+  grep, ls, read, list_sessions, list_propositions). Plan tree + 25 turn
+  records + a per-chat model override.
+- `chats/chat_mv293wv5_10656952.json` — the practice lifecycle (owner-built,
+  2026-10-10): graph-context (the bracketed preamble), problem ×2,
+  practice-thinking ×2, conclude ×2, practice-next, problem-dismissed — the
+  full marker cycle of a practice session, including the dismissal.
+- `graph/` — the real ledgers: nodes (282 lines), chains (160), sessions (8),
+  coverage (93), sources (4), render-settings, positions.
+- `jobs/` — five records covering kind × status: research done + aborted by
+  learner, demo done, visual done, pdf error.
+- `settings.json` — the stamped settings record (version 1, model slots).
+
+## Regenerating / extending
+
+Run the app (`npm run dev`), exercise the feature against `data/`, then copy
+the produced record in verbatim. Do not hand-edit content; if a record needs
+pruning, re-run the flow instead.