fix(check): allow missing caption overrides

[miguel-heygen]

Summary

Caption projects no longer fail check or deprecated validate when the optional root caption-overrides.json is absent. Only that exact resource's 404 and benign aborted fetch are ignored; a 500, another missing JSON file, or a malformed present sidecar remains an error.

The present-file contract is now explicit: the payload must be a JSON array whose entries are objects, and [] is the valid explicit no-op. Invalid JSON, a non-array root, or a non-object entry emits a runtime error that browser validation records instead of being silently ignored.

Related: #2263

Validation

• Caption override runtime suite: 14/14 tests passed.
• Validate and check-browser suites: 38/38 tests passed.
• Core/runtime and CLI package typechecks passed.
• Changed-file formatting, lint, and diff checks passed.

[miga-heygen]

Reviewed: caption-overrides.json 404 correctly treated as silent no-op in both validate and checkBrowser. Payload validation added with proper error reporting. Tests cover missing sidecar, malformed JSON, non-array, and non-object entries. — Miga