feat(editor): create-mode songs can hold several drum parts too

CHANGELOG.md

@@ -9,6 +9,93 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **Multiple drum parts now work when creating a new song too.** The several-
+  drum-parts feature previously covered only songs you re-open and Save; a
+  *create-mode* session (New Song from a Guitar Pro / MIDI import) was capped at
+  one drum chart. Now, once a melodic track exists, **＋ Track ▸ Drums** adds
+  another drum part in a create session just like an existing one, and Building
+  the song persists every part (the first as the song-level drum tab today's
+  game plays, the rest as `type: drums` arrangement entries per feedpak-spec
+  1.17.0). A drums-only draft still holds one part until you add a melodic
+  track (drums are never the primary chart). Packs stay backward-compatible.
+
+- **A song can hold several drum parts.** The payoff of the drums-as-arrangement
+  arc below: with drums already present, **＋ Track ▸ Drums** adds *another* drum
+  part (a second drummer, an aux-percussion layer), and a GP/MIDI drum import is
+  **added** as a new Drums track instead of replacing the one you have. Each part
+  is its own **🥁 entry in the part dropdown** — pick one to open *its* grid — with
+  its own Tracks row, mixer strip, and multi-track playback channel (two parts
+  hitting the same drum at the same instant both sound). Deleting a part removes
+  just that part (undoable, exact); renaming follows the part. Saving persists
+  every part: the first as the song-level drum tab today's game plays (packs stay
+  fully backward-compatible — older readers simply see that one), the rest as
+  `type: drums` arrangement entries per feedpak-spec 1.17.0, and they all come
+  back on reload. Create-mode compose sessions keep the one-part rule for now
+  (their build path persists a single drum tab).
+
+- **The drums track is now an ordinary mixer / Tracks channel.** Building on the
+  drums-as-arrangement work below, the drum chart's mixer strip and Tracks mix now
+  use the same per-arrangement channel address every other part does, instead of a
+  one-off "drums" slot. Mute / solo / volume on the drums strip behave exactly like
+  a pitched track's — including in multi-track ("play all") playback, where the drum
+  kit now follows its own strip — and the drum grid's guide claps follow that strip
+  too. With a single drum chart you won't see a difference (its durable track
+  identity is unchanged, so delete/undo, rename, and pairing all work as before);
+  this is the wiring that lets *several* drum charts each get their own strip.
+
+- **Drums are now a selectable part in the arrangement switcher.** Pick
+  **"🥁 Drums"** from the part dropdown to open the drum editor — exactly like
+  switching to Lead, Rhythm, or Bass. The drum chart is no longer a mode tucked
+  off to the side; it's a first-class part listed alongside the pitched ones (and
+  the 🥁 Edit Drums button and the Tracks row now keep the dropdown in sync).
+  Selecting a pitched part returns you to it as before — under the hood the drum
+  grid stays a view *over* the current pitched part, so nothing else about editing
+  changes. This is the visible step of the drums-as-arrangement work below, and the
+  set-up for having *more than one* drum chart to choose between.
+
+- **Drums are now a first-class arrangement (groundwork for multiple drum charts).**
+  The single drum tab has always lived *outside* the arrangement list as a lone
+  off-array singleton — the one instrument that wasn't an ordinary track. It now
+  also lives *in* the arrangement list as a `type:"drums"` arrangement, migrated
+  in automatically when a song loads. Nothing changes for you yet: the drum grid,
+  the Tracks row, the mixer, and saving all behave exactly as before (a saved pack
+  is byte-for-byte identical — drums still persist as the song-level drum tab), and
+  the drum editor's undo history is untouched. This is the load-bearing model
+  change that lets a song hold *more than one* drum chart (two drummers, an aux-
+  percussion layer) in a following release, built on the new authored-instrument
+  `type` so a drums track is identified by what it *is*, never its name.
+
+- **A track's instrument is now DATA, not a guess from its name.** The editor used
+  to infer whether a part was keys / bass / guitar from its *name*, in a dozen
+  places with two subtly disagreeing rules — the reason renaming a track could flip
+  its lane layout, and why the rename dialog has to refuse some names. The pack
+  format already records an authored instrument `type` per arrangement; the editor
+  now **reads it** (carrying it through load and save) and lets it drive *every*
+  identity decision — the keys/bass view, string counts, the guide-instrument
+  voice, the fretted-only Tab preview and Guitar-Pro export guards, and the
+  Parts-view silhouettes — falling back to the old name inference only when a
+  track is untyped. Existing songs open exactly as before — but a part named
+  against its instrument (a guitar called "Grand Piano") finally reads as, sounds
+  as, and exports as what it *is*. **The Tracks list now badges each transcription
+  track with its instrument** (GTR / BAS / KEY / DRM / VOX) instead of a generic
+  "MIDI", read from that same authoritative identity. And **renaming a typed track
+  is now free**: its identity is data, so the name is a pure display label and the
+  rename dialog no longer refuses a name that merely *looks* like another
+  instrument (untyped/legacy tracks keep the old kind-change guard, which protects
+  them from silently re-laning notes). Foundation for first-class drum tracks and
+  vocals.
+
+- **A per-track instrument-type control (next to the arrangement selector).**
+  Now that the editor honors an authored instrument `type` above the name for the
+  first time, an already-saved pack whose fretted (string+fret) chart has a keys
+  word *mid-name* — "Electric Piano", "Lead Synth" — can open **piano-locked**:
+  string buttons hidden, view stuck on the roll, with previously no way to
+  override it. The new **Guitar / Bass / Keys** dropdown sets the track's type
+  directly, which wins over the name everywhere — so a mis-named fretted part can
+  be corrected back to Guitar and reopens in string view immediately. The change
+  is undoable (Ctrl+Z) and persists with the pack on save; switching type rebuilds
+  the chart in place with no note loss.
+
 - **The piano roll stretches, compacts and scrolls.** Its lane height used to be
   derived and untouchable — the whole pitch range packed into about 350px — so a
   wide range collapsed to four pixels per semitone: passable for reading, useless

docs/USER-GUIDE.md

@@ -127,10 +127,10 @@ defaults** puts everything back.
 ## 3. Play and navigate
 
 - **Space** plays/stops from the playhead.
-- **Follow playhead** (`Shift+L`) keeps the view with the playhead during
-  playback. By default the view jumps ahead a page when the playhead reaches
-  the edge; turn on **View ▸ Scroll in Play** to pin the playhead and glide the
-  timeline under it instead (Logic's continuous-scroll manner).
+- **Follow playhead** (`Shift+L`) keeps the view with the playhead during
+  playback. By default the view jumps ahead a page when the playhead reaches
+  the edge; turn on **View ▸ Scroll in Play** to pin the playhead and glide the
+  timeline under it instead (Logic's continuous-scroll manner).
 - **Loop A/B** (`Alt+B`) compares the recording against your guide so you can
   hear whether the chart matches the take.
 - **Count-in** adds a bar of clicks before playback so you can catch the entry.
@@ -376,6 +376,13 @@ piano roll, with each row labeled by its GM note number (the familiar DAW
 drum-roll layout). The **drum limb lint** flags hits that would need three
 hands — advisory only.
 
+A song can hold **several drum parts** (a second drummer, an aux-percussion
+layer): with drums already present, **＋ Track ▸ Drums** adds another, and a
+GP/MIDI drum import is added as a new Drums track instead of replacing. Each
+part is its own 🥁 entry in the part dropdown — pick one to open *its* grid —
+with its own Tracks row and mixer channel. All parts save with the song; the
+first part is the one the game plays today.
+
 ---
 
 ## 8. Structure — sections, phrases, anchors, handshapes, tones