feat: add Sequence V2 format support to timeline editor

Implements full support for the sequence v2 DAG format with explicit
node routing and arrow syntax.

New features:
- timeline-format.js module for parsing/serializing v2 format
- NODE declarations with typed buffers (u8x4_norm, f32x4, etc.)
- Arrow syntax for effect routing: input1 input2 -> output1 output2
- Buffer chain visualization in properties panel and tooltips
- Node editor modal for adding/deleting node declarations
- Validation for undeclared node references (when NODEs explicit)
- Backward compatible with auto-inferred nodes

Files added:
- tools/timeline_editor/timeline-format.js (214 lines)
- tools/timeline_editor/test_format.html (automated tests)
- workspaces/test/timeline_v2_test.seq (test file with NODE declarations)

Files modified:
- tools/timeline_editor/index.html (~40 changes for v2 support)

All success criteria met. Round-trip tested with existing timelines.

handoff(Claude): Timeline editor now fully supports v2 format with
explicit node routing, NODE declarations, and buffer chain visualization.
Parser handles both explicit NODE declarations and auto-inferred nodes.
Validation only runs when explicit NODEs exist. Ready for production use.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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