feat(editor): add loop region playback toggle

[ChrisBeWithYou]

Summary

• add a toolbar Loop toggle for the selected timeline region
• keep loop controls synchronized with selected-region create, resize, clear, and disabled states
• restart editor playback at the region start when active playback reaches the selected end
• extend loop-region helper tests for normalization and playback restart behavior

Testing

• node --check screen.js
• node tests/loop_region.test.js
• node tests/pending_view.test.js
• node tests/eof_shortcuts.test.js
• node tests/scroll_bounds.test.js
• node tests/snap_options.test.js
• all tests/*.test.js via node

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/editor-loop-region-selector

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[byrongamatos]

Review pass (Claude Code) — stacked on #59; extends (doesn't conflict with) #47's loop-region strip, which is in its stack base. Clean: _normalizeLoopRegionPure swaps end<start, clamps to duration, enforces a min gap; _loopPlaybackRestartTimePure returns the region start once the cursor reaches the end; playbackTick seeks there via _restartPlaybackAt (and is suppressed while recording, so a take isn't disturbed); _updateLoopRegionControls auto-disables the loop when the region is cleared. node --check + all JS tests green (6 loop tests). No bug.

Note: only inherits the stack-wide items already flagged (built on the draft #47 loop strip + the pre-fix #55 drum-shortcut base). Nothing #60-specific. (Codex rate-limited until ~1 AM — manual + test-backed.)