Introduce a `ts type-equiv` Markdown fence: a verbatim paste of a source type
definition that `scripts/verify-type-equiv.ts` drift-checks against the source
symbol via the TypeScript parser, with provenance in a central
`scripts/type-equiv.manifest.json` kept 1:1 with the blocks. doc-typecheck
recognizes the same fence, skips compiling it (not standalone-compilable), and
excludes it from the opt-out ratio. Wired into the `doc-sync` chain.