Review stretch implementation across both toolsets

[mbreiser]

Summary

Stretch (per-frame timing multiplier) is not consistently implemented or visualized across MATLAB and web tools. Need a comprehensive review.

What to check

MATLAB side:

• PatternPreviewerApp — stretch display and animation timing
• Pattern_Generator.m — stretch value generation
• save_pattern.m / g6_save_pattern.m — stretch encoding in .pat files

Web side:

• pat-parser.js — stretch parsing from .pat files
• pat-encoder.js — stretch encoding
• pattern_editor.html — stretch UI (currently not exposed)
• arena_3d_viewer.html — playback timing using stretch values

Tasks

• Audit stretch storage format in .pat files (G4 vs G6)
• Verify stretch values are correctly preserved in round-trip (MATLAB → web → MATLAB)
• Add stretch controls to web Pattern Editor UI
• Ensure 3D viewer uses stretch values for accurate playback timing
• Document stretch value semantics (what does stretch=2 mean for timing?)