docs: the contract, the seam list and the @experimental tags name different interfaces #340

Context

Part of #56. Found by the 1.0 surface audit (#319), against main @ 550a41ef.

Three documents each list the extension interfaces 1.0 would freeze, and they
list different sets:

• docs/integration-contract.md:574-576 names EmbeddingProvider,
  CompletionProvider, AgentRunner, SkillEmitter "and the ui-sdk
  interfaces". It does not name SiteAdapter (packages/scrape/src/adapter/types.ts:66)
  at all, and "the ui-sdk interfaces" does not say which of the package's
  ~1,050 exported names (api-report/ui-sdk.txt) it means.
• docs/extending/README.md:62 says "All seams are marked @experimental until
  1.0". Of the nine seams it documents at :69-108, none carries its own tag:
  • the four core seams (packages/core/src/lib/seams.ts:22, :37, :54,
    :97) are covered only by a file-header comment at seams.ts:2, which a
    type checker and an editor tooltip never attach to the symbol;
  • AgentBackend (packages/ui-sdk/src/server/backend.ts:371), and the
    BackendBridge (:123) and BackendCapabilities (:51) it is made of,
    are untagged, while eight of their members and helpers are tagged
    (:98, :254-287, :335, :359, :422);
  • SpeechProvider (packages/ui-sdk/src/server/speech.ts:20), AsrClient
    (packages/ui-sdk/src/client/asr.ts:15), ToolRenderer and RendererPack
    (packages/ui-sdk/src/client/renderers.ts:44, :68) and SiteAdapter
    are untagged. Only BackendModule (packages/ui-sdk/src/server/backend-module.ts:151)
    is tagged the way the Stability section says every seam is.
• Nothing checks that the three agree, so they drifted apart one additive
  change at a time.

The same pass found three stale statements in the documents 1.0 would freeze:

• packages/ui-sdk/README.md:25 says "Protocol rev 3"; PROTOCOL_REV is 4
  (packages/ui-sdk/src/protocol.ts:44).
• The brain jobs scrape --json example at docs/integration-contract.md:135
  and :172 uses remoteineurope, the board 97d19d9 retired.
• The Consumers table (docs/integration-contract.md:25) says brain.db is read
  for voice keyterms; packages/ui-server/src/graph/reader.ts also reads the
  graph_* tables the brain.db section documents.

Which exports belong in the frozen set beyond the nine documented seams (the
permission-gate, confirm-pattern and subprocess-env helpers, VOICE_ALLOWED_TOOLS,
DocumentRenderer, the module manifest) is a judgement. It is not part of
this issue. #343 holds those questions.

Scope

• Put an individual @experimental TSDoc tag on each declaration of the nine
  documented seams, and on the types a seam is made of (BackendBridge,
  BackendCapabilities, StartTurnRequest, RendererPack, SpeechSession,
  AsrClientOptions, AdapterResult, ScrapeContext). The per-symbol tags
  replace the file-header comment in seams.ts.
• Rewrite docs/integration-contract.md "Extension interfaces" to name every
  seam and the package that exports it, instead of "the ui-sdk interfaces".
• Fix the three stale statements above.
• Add a test that reads the seam list in docs/extending/README.md, the list in
  docs/integration-contract.md "Extension interfaces", and the
  @experimental tags on exported declarations, and fails naming any seam
  that appears in one and not the others.

Out of scope

• Deciding whether the tagged-but-undocumented helpers stay in the frozen set.
  That is docs: decide what 1.0 freezes where the contract document does not say #343. This issue tags only what is
  documented today.
• Changing any interface's shape.
• Removing @experimental anywhere. That happens at 1.0.

Acceptance criteria

• Every declaration listed under Scope carries its own @experimental tag,
  and seams.ts no longer relies on a file-header comment.
• docs/integration-contract.md "Extension interfaces" names each of the
  nine seams by interface and package.
• packages/ui-sdk/README.md states rev 4; the scrape example names a board
  that exists; the Consumers row names the graph reader.
• The new test fails when the tag is removed from one seam (for example
  SiteAdapter), and when a seam is deleted from either document. Each
  mutation and its failing assertion is recorded in the PR.

Verification

bun run test including the new test; the mutation receipts in the PR.

Contract impact

• This touches the integration contract, so it needs the contract label, a CONTRACT: commit prefix, and the doc updated in the same commit.
• This changes a published package's behaviour, so it needs a changeset.

The contract edit is wording and classification only: no shape changes.