Stream drum gameplay metadata (feedpak 1.20.0) on the highway WebSocket #1104

What
feedpak 1.20.0 (FEP #67, feedpak-spec#68) lets a drum_tab.json carry drum gameplay metadata: per-hit a (accent) and x2 (double kick) flags, star_power / fills / solos ranges and easier difficulty levels (easy|medium|hard: {hits, generated}). The chart importer in #1091 writes them. The highway WebSocket does not pass them on: lib/drums.py hit_to_wire drops a and x2, and the drum_tab frame carries neither the range lists nor the levels, so bundle.drumTab never has them.

Why
A drums renderer (the drums plugin) needs them to score a chart the way a rhythm game does: Expert vs Expert+ (x2 kicks), Easy/Medium/Hard, star power phrases and fill activation, solo sections, and authored accents for the dynamics bonus. Today it can only draw the full hits[].

Proposal
Additive wire change: drum_hits keep a / x2; the drum_tab frame gets star_power, fills, solos (when non-empty) and a levels index {id: {total, generated}}; each level's hits stream after the top-level ones as drum_level_hits chunks ({level, data, total}, 500 per frame). highway.js stores everything on bundle.drumTab. A tab without the keys streams byte-for-byte the old frames; older clients ignore the new frame type.

Version: 0.3.0-alpha.1 (on top of #1091), any platform.