v4.9.0 — universal slug + XRef crash & build-labels slug-404 fixes

Minor release. Makes the optional slug URL-override field universal across all profiles, and fixes two latent bugs surfaced by the mathematical-guides / claude-books consumers:

• fix(XRef.astro): the frontmatter doc comment's {/* */} example, inside a /** */ JSDoc block, closed the block early — esbuild threw Unexpected "}" on any MDX import of <XRef>. Now // line comments; the MDX-correct example is restored.
• fix(build-labels): cross-reference hrefs now prefer frontmatter slug: over the filename, matching Astro's entry.id (no more 404s under a custom slug).
• feat(schemas): slug added to academic / tools / minimal / course-notes (research-portfolio already had it).

288 tests (was 275). Full notes in CHANGELOG.md.

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