fix(lint): ignore embedded caption rail entrypoint

[miguel-heygen]

Summary

• ignore the root-level rail.html helper emitted by the embedded-captions theme workflow when detecting multiple root compositions
• retain the existing caption-skin.html exception
• add a regression test covering index.html plus the generated rail entrypoint

Reproduction

An embedded-captions Standard theme writes both index.html and rail.html at the project root. hyperframes check treated both as standalone compositions and emitted multiple_root_compositions, even though render-and-composite.sh renders each in an isolated shadow project.

Verification

• bun run --filter @hyperframes/lint test (362 tests)
• bun run --filter @hyperframes/cli test -- --run src/utils/lintProject.test.ts (76 tests)
• lint and CLI typechecks
• oxlint and oxfmt checks
• pre-commit hooks