Audit stripMarkdown for production-path necessity

[RE-codes]

Context

src/slug.ts exposes stripMarkdown, which removes ATX # markers, inline code, image syntax, link wrappers, HTML tags, emphasis markers, emoji, and (as of issue #19) trailing setext underlines from a markdown heading source string.

During issue #19 we verified via DevTools that Obsidian's HeadingCache.heading already returns the plain heading text — underline stripped for setext, and inline-formatting also appears stripped based on the existing paired fixtures (e.g., ## **Bold Heading** is reachable in test-native.md via #Bold%20Heading, suggesting the cache stores "Bold Heading" not "**Bold Heading**").

If the cache already does this work, every production call site (resolver.ts:40, resolver.ts:45, readingModeHandler.ts:39) is feeding pre-stripped text into a stripper. The function is effectively dead code in production; only the unit tests exercise its branches.

Tasks

• Inspect app.metadataCache.getFileCache(file).headings in the dev vault for every formatted-heading shape in test-gfm.md / test-native.md: bold, italic, code, image, link, HTML, square-bracket, escaped, setext, emoji. Record what the cache returns for each.
• Decide one of:
  • Trust the cache. Reduce stripMarkdown to a trim (or remove it). Update callers and tests. Document the cache contract relied on.
  • Keep stripMarkdown as a defensive boundary. Document explicitly why — e.g., planned public API, observed cache leakage in some shape, or platform variance.
• Update or remove unit tests in src/slug.test.ts consistent with the chosen direction.
• Update plan/observed-behavior-matrix.md and plan/QA-log.md if behavior expectations change.

Acceptance Criteria

• app.metadataCache behavior for every heading shape currently exercised by fixtures is documented.
• slug.ts either trusts the cache (smaller, simpler) or carries a comment explaining why it doesn't.
• Tests align with whichever direction is taken.
• npm test passes.

Out of scope

• Adding new heading-shape coverage (separate child issues in the P2a epic).
• Changing the slug algorithm itself.

References

• Issue P2a heading: Setext heading #19 (setext heading) — PR thread documents the production-path tracing that surfaced this concern.
• src/slug.ts — current stripMarkdown implementation.
• src/__tests__/fixtures/test-gfm.md, test-native.md — existing paired-fixture coverage.