feat(core): publish an ./extraction subpath — html→markdown for the SD7 capture seam

[KnockOutEZ]

Closes #402.

What changed

wigolo/extraction is now a published subpath carrying exactly two names:

• htmlToMarkdown (src/extraction/markdown.ts)
• extractMetadata (src/extraction/metadata.ts)

The full extraction graph already shipped in dist/, but no legal specifier reached
any of it — 16 subpaths, no wildcard, and none of them re-exporting a markdown symbol.
SD7's capture-on-settle seam (wigolo-studio-run#365, packages/studio-core) has the
page's HTML from SessionDrive.readCurrentPage() and no way to turn it into the
markdown body recordVisit stores.

Two named functions rather than pipeline.ts: the seam runs on the navigation path in
the DB-owning child, so the provider/registry graph — and a native module with it —
must not arrive there for a string conversion. src/extraction/index.ts is the barrel;
markdown.ts pulls turndown + lang-hints, metadata.ts pulls linkedom, and nothing
else.

Zero behaviour change: two export lines, one exports entry, one test entry.

Verification (local; CI deferred — quota)

• npm test — 934 files, 11,543 passed / 20 skipped / 7 todo, 0 failed
• npx tsc --noEmit — clean
• npm run gate:studio — green (tests/ type-check debt holds at baseline 341)
• npm run build — clean; dist/extraction/index.{js,d.ts} emitted

The exports pin grows the subpath with both halves the A4 rule demands (declared target

• a real import() in a child Node process), and with an EMPTY types set — a pin, not
  an omission, so the pipeline's types cannot arrive later without a measured import site.

Mutations run and restored byte-identical:

• drop extractMetadata from the barrel → reds the runtime probe AND the .d.ts pin
  (expected [ 'htmlToMarkdown' ] to deeply equal [ 'extractMetadata', 'htmlToMarkdown' ])
• point the exports entry at a path that is not on disk → reds the declared-target arm
  and fails the real import() (ERR_MODULE_NOT_FOUND), which is the exact lie a
  resolve-only probe would have told

Fresh install of built core (npm pack → clean dir):

17 subpaths: . ./types ./companion-broker ./cache/db ./config ./account ./companion
./companion-contract ./security ./fetch-tiers ./extraction ./cache ./cache/library
./embedding-queue ./search-tokens ./logger ./daemon

wigolo/extraction keys: [ 'extractMetadata', 'htmlToMarkdown' ]
markdown: "# Hi\n\na [link](https://x.dev)"
title: "Settled page"

Territory

lane-extra: package.json and tests/unit/package-exports.test.ts — a subpath cannot
be published without the exports map, and the pin for it is an exact list that travels
with that map. Both are named in the issue's own files hint; same pair as
wigolo-studio-run#363/#370.

CI deferred — quota.

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