docs: Add low-priority tasks for tempo control and asset debugging

Added two future enhancement tasks:

Task #65: Data-Driven Tempo Control
  - Move g_tempo_scale from hardcoded main.cc to .seq or .track files
  - Approach A: TEMPO directive in .seq (time, scale pairs)
  - Approach B: tempo column in music.track
  - Benefits: Non-programmer friendly, easier iteration
  - Priority: Low (current approach works, but less flexible)

Task #66: External Asset Loading for Debugging
  - Load assets from files via mmap() instead of embedded arrays
  - macOS only, non-STRIP_ALL builds
  - Benefits: Edit assets without rebuilding assets_data.cc (~10s saved)
  - Trade-offs: Runtime file I/O, development-only feature
  - Priority: Low (nice-to-have for rapid iteration)

Both tasks target developer workflow improvements, not critical for 64k goal.

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