[dashboard-language] Built-in pages: validate DLS-PAGE-014 data-state exposure

Caution

Protected files were modified in this change.
This pull request is in request_review mode and requires explicit human scrutiny before merge.

Protected files: README.md

Milestone

Built-in pages — DLS-PAGE-014 conservative built-in data-state exposure validation.

Added conservative implementation-local built-in page definition support for definition.data-state.
Validated that built-in page definitions declare independent availability, completeness, and freshness exposure markers.
Rejected unknown built-in data-state axes and non-canonical marker values.
Expanded validator unit coverage for accepted and rejected DLS-PAGE-014 cases.
Updated PLAN.md and README.md to record the slice and the remaining ambiguity in the specification vocabulary.

npm install ✅
npm run typecheck ✅
npm run lint ✅
npm test ✅
npm run test:e2e ⚠️ blocked: Playwright Chromium executable is not provisioned in this environment (browserType.launch: Executable doesn't exist)

End-to-end browser tests remain blocked by missing Playwright browser provisioning in the runner.

Built-in pages — next conservative Section 10 slice for runs page link coverage without inventing presenter semantics.

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