docs: expand examples page with per-track piano roll and new generated examples

[DaoTwenty]

Summary

• Render script: replaced the single shared piano roll with per-track horizontal lanes; context tracks use palette colors, generated tracks render in gold with a GEN badge; controls text moved to its own header row to prevent overlap with the task label
• examples.md: fully rewritten with 5 curated examples — Californication drums (×3 density), Bach BWV 1052 cello (×5 duration levels), Black Magic Woman guitar solo (×2 resamples, model-sampled controls), SSBM All Star harp bar infill (×2 duration), Summer multi-step composition (continuation + track sampling); removed Hotel California, HOME Resonance, and Summer AR duplicates; every example includes a track table with exact INSTRUMENTS.md names and Context/Generated roles
• New MIDI + video assets: 14 new MIDI files and 13 new MP4s; all 22 videos re-rendered with Arachno SoundFont for proper GM instrument audio
• SSBM All Star: harp seed bars (0–3) and generated bars (4–7) merged into a single track so they appear on one piano-roll lane with the generated region shaded

Test plan

• mkdocs serve — verify all videos and download links resolve correctly
• Check tab switching works on cello (5 tabs) and Californication (3 tabs) examples
• Confirm per-track lane layout renders correctly in the piano roll videos