docs: correct stale examples in phase-2-headless-adapter-development.md

[Copilot]

Three locations in docs/plans/phase-2-headless-adapter-development.md described an earlier design that was superseded when HeadlessRenderProvider was refactored and HeadlessAudioPlayer methods were renamed.

Changes

• HeadlessAdapter class summary (line 122) — replaced "Records all provider calls for verification" with the accurate description: providers expose their own typed recording collections.

• HeadlessRenderProvider example block (lines 184–220) — replaced single List<object> _recordedCommands approach with current three-list implementation:

  private readonly List<SpriteDrawDto> _recordedSprites = [];
  private readonly List<TextDrawDto>   _recordedTexts   = [];
  private readonly List<MeshDrawDto>   _recordedMeshes  = [];
  
  public IReadOnlyList<SpriteDrawDto> RecordedSprites => _recordedSprites;
  public IReadOnlyList<TextDrawDto>   RecordedTexts   => _recordedTexts;
  public IReadOnlyList<MeshDrawDto>   RecordedMeshes  => _recordedMeshes;

• Technical Details prose (lines 90 & 94) — updated HeadlessRenderProvider description to reference typed lists (RecordedSprites, RecordedTexts, RecordedMeshes); corrected HeadlessAudioPlayer method names from Play/Stop → StartPlayback/StopPlayback.

PR Impact

Documentation only — no source changes, no API or behaviour impact.

PR Size justification

Three targeted line-level edits to a single plan document.