fix(quality): validate typecheck baseline schema (#12419)

The API-route and Open-SSE typecheck gates treated every top-level baseline value as a diagnostic map, so policy metadata such as _relax_velocity_2026_08_30 was iterated character by character and reported as fabricated numeric TypeScript improvements. Both gates now share one fail-closed baseline boundary: underscore-prefixed top-level keys are reserved for metadata and skipped, and real file entries must be plain objects.

Worth a follow-up: check-dashboard-typecheck still prints the same fabricated entries, so it looks like a third gate with the same defect that this PR's scope does not cover.

Validated in a combined worktree with the batch's ready set boarded onto the current tip: parse sweep clean on every changed TypeScript file, typecheck:core clean, check:dashboard-typecheck OK (207 pre-existing errors, all within baseline), check:cycles OK, check-file-size OK, 203/205 focused node tests and 94/94 vitest — the two failures belong to #12427, which is held back.