Add PUL-F010 regression anchors for composition+index URL parameter

[Brad-Edwards]

Summary

PUL-F010 — when the index URL parameter is present alongside a
composition, the runtime SHALL position playback at the given
zero-based index within the composition.

The runtime path that satisfies PUL-F010 already shipped end-to-end
with PUL-F008 (PR #70) and was reinforced for PUL-F009 by PR #72.
This PR pins PUL-F010's distinctive contract — the index parameter
positions playback at the zero-based composition entry — at the
loader boundary, following the regression-anchor precedent set by
the PUL-F009 PR (tests/runtime/scene-loader.test.ts:177).

What changed

• tests/runtime/scene-loader.test.ts — new describe block
  'composition + index positions playback (PUL-F010)' with two
  regression anchors:
  • ?composition=trailer&index=2 against a 4-entry manifest selects
    manifest[2] as the head and slices forward (proves zero-based
    positional semantics for N > 1; distinguishes from PUL-F009's
    index=1 anchor).
  • Two sequential loader.handle() calls against the same
    composition with index=0 then index=2 resolve to different
    head scenes (proves the index parameter — not the composition
    id — decides where in the composition playback starts).
• docs/adrs/013-url-navigation-grammar-boundary.md — extends ADR-013
  to name PUL-F010 explicitly: index is not scene identity, an id
  alias, or a persistent bookmark; adds the "PUL-F010 forks from the
  existing composition path" risk + mitigation.
• docs/adrs/014-url-scene-target-selection.md — records that
  composition + index is PUL-F010's positional navigation
  behavior, that the head scene is manifest[index], and adds the
  "index handling creates a second playback state machine" risk +
  mitigation.
• CHANGELOG.md — entries for the new anchors and the ADR updates.

No production code changes. The runtime contract PUL-F010 names was
already satisfied by the dispatcher (resolveSceneNavigation →
resolveCompositionAndIndex in src/runtime/scene-navigation.ts)
and the loader (createSceneLoader in src/runtime/scene-loader.ts).

Test plan

• pnpm lint — clean
• pnpm typecheck — clean
• pnpm test — 510 tests pass (12 files; scene-loader spec
  grew from 19 → 21 tests with the two new anchors)
• pre-commit run --all-files — clean
• gc_codex_review (uncommitted, cycle 1) — 0 findings (core
  reviewer + security reviewer)

Closes #19

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Brad-Edwards]

gc_codex_review cycle 1 of 2 complete for PR #73. Posted by the MCP server to enforce the hard-cap-2 contract (issue #794). Do not edit or delete — used by the next gc_codex_review invocation to count cycles.