Add debug file-source overlay while serving

[lreading]

Summary

Post-v1 backlog item: add a debug-only overlay during serve that shows which source file or data is driving the current page.

This should help authors connect rendered slides back to YAML while editing. It must never appear in production builds.

Alternatives considered

Docs-only guidance is simpler, but an in-browser debugging aid could make authoring much easier.

Affected packages

• app (presentation renderer)
• cli (@slide-spec/cli)
• docs (documentation site)
• shared (types and validation)
• Unsure