feat(view): Rock Band-style 3D drum highway with keyboard drumming

CHANGELOG.md

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+# Changelog
+
+All notable changes to the Drum Highway plugin. The format follows
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/); versions follow `plugin.json`.
+
+## [Unreleased]
+
+Version 4.2.0 in `plugin.json` (4.1.0: scoring, 4.2.0: the 3D drum highway).
+
+### Added
+- **Rock Band-style 3D drum track** (`assets/highway3d.js`, on core's vendored three.js): pad lanes and a
+  kick bar, bevelled rectangle pad gems and domed cymbal gems, rectangular strikeline targets, hit flashes
+  and sparks, star power and fill highlighting, drum-solo tint, and a HUD with streak, multiplier, star
+  power, score, stars, accuracy and the difficulty badge. Scored by the same session as the 2D view.
+- **View** setting — Auto (3D with WebGL2) / 3D / 2D; the main player swaps renderers on change.
+  `feedBackViz_drums3d` / `feedBackViz_drums2d` for hosts that want one view.
+- **Keyboard drumming** in both views (**Keys**, on by default): B kick, F red, J/K/L toms,
+  Shift+J/K/L or U/I/O cymbals. Enter activates star power on charts without fills (in both views, with or
+  without Keys; it is left alone when star power can't be activated).
+- The 3D HUD carries the early/late **timing meter** and its **Apply** button, the star power hint and the
+  "This song has no drums" message; gems move on the smoothed highway clock the scoring uses.
+- **Rock Band-style scoring** from a JavaScript port of YARG.Core's drums engine (`assets/engine.js`,
+  LGPL-3.0, see `NOTICE.md`): ±70 ms hit window, multiplier up to x4 (x8 in star power), pro-drums
+  cymbal/tom lanes, accent/ghost dynamics bonus, stars, and star power with fill activation. Driven
+  through `assets/session.js`, which also adds spot-on timing, roll and drum-solo bonuses. The 2D view
+  marks hits / misses from the engine and shows a score HUD when **Score** is on.
+- **Difficulty per player** — Easy, Medium, Hard, Expert, Expert+ (`drums_difficulty_v1`), from the
+  settings panel, the HUD badge or D / Shift+D; Expert drops the double-kick notes. Easier levels, star
+  power, fills and solos come from the drum tab's feedpak 1.20.0 gameplay metadata; without it the song
+  plays Expert (plus Expert+ when the chart has double-kick notes, e.g. a legacy chart with GM 35 kicks),
+  without star power.
+- **Assists** — auto kick / auto cymbals up to a chosen difficulty (played on the drum sound at their own
+  volume), hit-timing presets (Relaxed / Forgiving / Normal / Precision), **Pro cymbals** toggle and an
+  input **Offset** (ms) for audio / MIDI latency.
+- **Timing meter** under the score: the last 24 hits early / late on a ±100 ms bar, their median, and the
+  input offset that would centre them, with an **Apply** button.
+- **Star power without fills**: on charts with star power phrases but no drum fills, press **Enter** or
+  hit the **SP pad** (settings: off, hi-hat pedal, ride bell, splash or china; `drums_sp_pad_v1`) to
+  activate it. With fills, the note that ends a fill activates it, as before.
+- **Results card** at the end of the song (single view, Score on): total score with the bonuses, best
+  score per song and difficulty (`drums.best:<song>|<difficulty>` in `localStorage`), stars, accuracy,
+  notes hit, best streak / full combo and where the points came from.
+- The drum view on a part that isn't drums (no drum tab, arrangement not named Drums / Percussion) shows
+  "This song has no drums" or "Pick the Drums part" and plays / scores nothing, instead of reading guitar
+  notes as drum notes.
+
+### Changed
+- Hit timing: the view draws and scores on a smoothed per-highway clock (interpolated between the
+  player's audio samples, like core's 3D highway), a pad hit is timed when the MIDI message arrived
+  (its timestamp, when the MIDI provider passes one) rather than at the last frame, and the engine runs
+  60 ms behind the frame clock so a hit struck just before a frame is judged at its own time instead of
+  being moved forward to the frame.
+- The input offset applies to the engine's clock as well as to the hits, so a positive offset no longer
+  leaves a late-registered hit facing a note the engine has already missed.
+- MIDI velocity reaches the engine as 0..1 (velocity / 127); velocity 1 used to count as full strength.
+- Auto kick / auto cymbals are timed from the audio clock (mapped to the stems plugin's audio context
+  when it is there), scheduled 100 ms ahead by one forward-only schedule shared by all drum views and
+  cancelled on pause / seek: no more late or doubled auto notes.
+- Splitscreen: kit hits go to the focused drum view if it is on screen, else to the drum view that is,
+  so a hidden main-player drum view no longer swallows them.
+- A scoring file that failed to load is loaded again on the next try (its dead `<script>` element is
+  removed).
+- A drum-tab hit is an accent only when the tab marks it (`a`, feedpak 1.20.0) or, for older tabs, when it
+  has a written velocity of 100 or more; a hit with no velocity is no longer drawn as an accent. A kick
+  marked `x2` is a double kick (GM 35).
+- The "Hits" checkbox is now **Score**.