Fix file metadata aliases blocking collection discovery

[callumalpass]

Summary

• Use schema-valid file_mtime and file_path selection aliases.
• Read manuscript modification times from the corrected metadata alias.
• Add regression coverage using the installed query schema for configured, starter, and unconfigured collections.

Validation

• Writer Vitest: 168 tests passed before applying to latest main.
• pnpm typecheck passed.
• Full pnpm test stopped on a local sibling Reader resource-version mismatch; CI validates in isolation.

Requested hotfix for production collection and annotation index loading errors.

[callumalpass]

Deployed merged commit fcdd727204528ec7f0efabf7b51a397c9c8b1ba7 through the documented local clean-tree deploy script (GitHub environments/secrets are not yet configured).

• PR CI passed: checks, production build, baseline and reliability Chromium suites.
• Staging: https://b39d97f4.mdbase-writer.pages.dev (alias https://staging.mdbase-writer.pages.dev).
• Production: https://7f035e4b.mdbase-writer.pages.dev (custom domain https://writer.mdbase.dev).
• Both targets: verified manifest origin, fetched entry bundle hash against local build, corrected file_mtime/file_path aliases, and unauthenticated landing-page rendering without uncaught browser errors.
• No connected-collection acceptance test was performed against production data.
• Canonical checkout is clean on main.