fix: dedup heading-anchor IDs and add controls to reduced-motion demo videos

[tomasz-tomczyk]

Summary

Two review-page / marketing fixes found in the pre-release audit of v0.9.0..main:

• Duplicate heading IDs — document-renderer.js heading_open set id=slug with no de-duplication, so a doc with two headings slugifying to the same value emitted duplicate ids and anchor clicks (hashchange nav and the in-page heading_close link) always scrolled to the first match. Ports crit local's GitHub-style dedup (baseSlug, baseSlug-1, …) via a counter on the md instance, reset once per buildLineBlocks render so it doesn't leak across re-renders. IDs now match crit local exactly (parity contract).
• Reduced-motion demo videos — the homepage mode-grid videos had only data-lazy-src (no src) and no controls; under prefers-reduced-motion app.js skips loading them, leaving a poster with no way to play. Adds controls to match the hero and mode-page videos.

Review

• Code review: passed (fresh frontend review; dedup reset semantics + both anchor consumers verified)
• Parity audit: passed (id scheme byte-identical to crit local; per-file reset matches parseMarkdown clear)

Test plan

• mix precommit: 1015 tests, 0 failures
• Playwright chromium E2E: 131 passed
• See also: fix(preview): preserve comment DOM anchors across fetch + re-share #244 (DOM-anchor round-trip fix from the same audit)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.42%. Comparing base (21a90e5) to head (2837a52).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
=======================================
  Coverage   82.42%   82.42%           
=======================================
  Files          89       89           
  Lines        3089     3089           
=======================================
  Hits         2546     2546           
  Misses        543      543           

Flag	Coverage Δ
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