fix(ui): in-page anchor navigation for headings

[dgrissen2]

Summary

• Intercept #-prefixed link clicks in InlineMarkdown and smooth-scroll
  within the sticky scroll viewport. The native browser jump didn't work
  because plan content lives inside a scrollable container, not the
  document root.
• Expose both a canonical slug and a legacy slug per heading via
  data-anchor-aliases so previously shared URLs keep resolving. Canonical
  collapses runs of separators; legacy preserves them.
• Handle hashchange and the initial hash so direct links land on the
  right heading after load.
• New packages/ui/utils/anchors.ts with normalizeAnchorText,
  decodeAnchorHash, slugifyHeadingAnchor, legacySlugifyHeadingAnchor,
  getHeadingAnchorAliases + unit tests.
• Thread onNavigateAnchor through BlockRenderer, AlertBlock,
  Callout, TableBlock, TablePopout, proseBody.
• Ignore .serena/ tooling state.

Test plan

• bun test packages/ui/utils/anchors.test.ts
• Render a plan with ## Part I: Overview: Scope and click a TOC link
  like [Scope](#part-i-overview-scope) — scrolls smoothly, sticky
  header isn't overlapping the target.
• Open a shared URL that uses the legacy double-hyphen slug
  (#part-i--overview--scope) — still resolves via alias.
• Load a page with #some-heading in the URL directly — auto-scrolls
  on first render.
• Anchor links inside tables, callouts, alerts, and blockquotes all
  navigate in-page.