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perf(render-metadata): snapshot heading slugger occurrences (#337) (#348)

* test(render-metadata): add red tests for heading slugger snapshot (plan 006)

Red/TDD phase for plan 006 (heading-id dedup O(N^2) -> O(tail)).

- Equivalence guard: chunked duplicate-heading stream yields byte-identical
  ids to a single-shot parse (passes against current replay code).
- Scan-count scaling guards spy Slugger.prototype.slug and assert slug()
  calls are proportional to the divergent tail, not the full heading count
  (0 on non-heading append, 1 per new heading, 4 across 4 flushes). These
  fail against the replay path and go green once the occurrences
  snapshot/restore lands (Step 2).

Refs #337

* docs(guard): log 006 checkpoint 1 — red tests verified ON TRACK

Red-phase checkpoint for plan 006. Equivalence guard green as the plan
prescribes; three scan-count guards fail for the right reason (missing
occurrences snapshot/restore in render-metadata.ts). Snapshot c0f5fdc.

* perf(render-metadata): snapshot heading slugger occurrences (plan 006)

Make streaming heading-id dedup O(tail) instead of O(H) per flush.

On an append-only pass, restore the github-slugger `occurrences` map
from a snapshot captured at the stable-prefix boundary instead of
replaying `slug()` over every prior heading. A parallel
`preparedHeadingSnapshots` array stores {offset, occurrences-clone}
alongside each prepared heading; the tail is slugged against the
restored map and re-snapshotted for the next pass.

Correctness safeguards:
- Restore only when the header-id/prefix signature matches and every
  prefix heading's offset lines up with its stored snapshot offset;
  otherwise fall back to the existing full replay path.
- Output is byte-identical to replay (prefix headings keep their
  previously-computed ids; tail slugged against restored occurrences).

Refs #337

* docs(guard): log 006 checkpoint 2 — green phase ON TRACK

Step 2 (occurrences snapshot/restore) verified against snapshot... (continued)

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