Docs: add SEPT_VERIFY.md, the DAW checklist for Aug-Sept work

docs/SEPT_VERIFY.md

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+# September DAW verification — new work not yet hand-tested
+
+Checks for work landed or in progress since the last `VERIFICATION.md` pass that no unit test can
+close. Build **Release**, install into `UserPlugins/`, restart REAPER. Panel tail toggle = **None**,
+project rate 48000, unless a check says otherwise.
+
+## 1. Instrument control-surface pass (landed Aug 3, 2026)
+
+- [ ] Play notes and watch the ReaSampler 9000 editor's master meter — it redraws smoothly (~60 fps)
+- [ ] Load a heavy project (several other plugins, high track count) and watch the meter again under
+  that load — still smooth, no visible stutter
+- [ ] Click the single-button "Envelope" toggle on a deck — cycles on/off cleanly; confirm its
+  disabled-state look when the deck it governs isn't applicable
+- [ ] Same check for the "Filter" toggle
+- [ ] Same check for the "Limiter" toggle
+- [ ] Same check for the "Stage"/"Spline" toggle (the envelope mode selector — one button reading
+  either "Stage" or "Spline")
+- [ ] Click anywhere on an envelope deck's control section (not just a knob) — that envelope becomes
+  selected for editing on the waveform overlay
+- [ ] Switch the instrument to Trigger mode — the loop markers and their caption disappear from the
+  waveform entirely
+- [ ] Switch to Gate mode with looping off — the loop markers still show, greyed out, and are still
+  grabbable
+- [ ] Zoom in on a loop marker and drag it near a zero crossing — it snaps only within a small pixel
+  radius of the crossing, not from anywhere on the waveform
+- [ ] Hold Ctrl while dragging a marker near a zero crossing — snapping is disabled for that drag
+
+## 2. Bank panel additions (Aug 3)
+
+- [ ] With a track selected and a bank card focused, click "Insert as FX" — ReaSampler 9000 loads on
+  that track with the focused sample already loaded
+- [ ] Open the panel's top-toolbar "⋯" overflow menu — Re-capture and Insert Conform are both there
+  and both work
+- [ ] Drag a bank card over the arrange view — the cursor shows a crosshair cue while over a valid
+  arrange-insert target
+
+## 3. Design View FX parking (Aug 21)
+
+- [ ] Trigger a mode switch that parks/restores FX on a heavy chain, and watch the docked bank
+  panel while the reclaimer drains: the cursor goes busy over the panel and the footer reads
+  "parking FX" / "restoring FX" appropriately. If the cursor never changes, note it — REAPER may not
+  route `WM_SETCURSOR` to the docked panel at all (`src/shell/panel/CLAUDE.md` Gotchas)
+- [ ] Native drag-reorder a parked track's FX chain, switch modes, and confirm each plugin's state
+  restores to the SAME plugin it came from (not swapped by slot)
+- [ ] Do the same reorder via an SWS move command
+- [ ] Save the project after a reorder, reopen, then switch modes — restore still keys to the right
+  plugin
+- [ ] Reorder a chain holding two copies of the same plugin — each copy's state restores to itself,
+  not swapped with its twin
+  (`docs/TODO.md` §"FX-GUID stability for the park's restore is unverified in the DAW")
+- [ ] For mode-switching itself (solo-set restore, transport refusal, persistence, undo point count,
+  etc.) — see `docs/VERIFICATION.md` §"Mode switching"; not repeated here
+
+## 4. Drag a card into a plugin's editor (in progress, Sept 2026)
+
+- [ ] Drop a bank card on Serum 2's oscillator in its floating FX editor window — Serum loads the
+  sample
+- [ ] Drop a bank card on Serum 2's oscillator from the FX chain window — same, Serum loads the
+  sample
+- [ ] Drop a bank card on the FX chain list or window chrome (not the oscillator) — ReaSampler 9000
+  loads as before, unchanged
+- [ ] Drop a bank card on the TCP, MCP, or arrange — unchanged from current behavior
+- [ ] Start a drag over a plugin editor, then move back over the arrange mid-drag — the drag reverts
+  to the panel's own drop behavior (may only visibly update on the next key press or mouse-button
+  change)
+- [ ] Drag a card out to Windows Explorer and back into REAPER — control hands back to the panel's
+  own drag correctly
+- [ ] Drop a card onto an already-open ReaSampler 9000 editor — it shows its "drop on the panel"
+  banner and does NOT spawn a second instance
+- [ ] Drop a card onto ReaEQ's editor — shows a no-drop cursor, nothing loads
+
+## 5. Live spline-envelope drags (in progress, Sept 2026)
+
+- [ ] Set the amp envelope to Spline, hold a note, and drag a knot — the sound audibly follows the
+  shape change during the drag itself
+- [ ] Same check for the pitch envelope
+- [ ] Same check for the filter envelope
+- [ ] After dragging a knot, trigger a new note — it uses the dragged shape
+- [ ] Listen closely during a fast drag for any audible glitch beyond what an ordinary knob move
+  would cause (clicks, zippering)
+- [ ] Drag a knot on a held note and listen for an audible pitch/amp/filter glide as it moves, not a
+  stepped jump
+- [ ] Release a dragged knot — commits exactly as before: one undo point, and reopening the project
+  keeps the dragged shape
+- [ ] With a note held, add a new point to the spline — audible immediately
+- [ ] With a note held, delete a point from the spline — audible immediately
+
+## 6. Sample-rate playback (in progress, Sept 2026)
+
+- [ ] In a 48 kHz project, import a 44.1 kHz file via the Media Explorer into ReaSampler 9000; play
+  its root note — same pitch and length as the file auditioned in the Media Explorer. Repeat in
+  Preserve mode.
+- [ ] Same with a 96 kHz file in a 44.1 kHz project
+- [ ] Envelope attack times, loop (a single-cycle loop holds its pitch; loop crossfade length) and
+  filter cutoff sound the same as on a same-rate copy of the file
+- [ ] Bake a mismatched-rate sample — the baked result has the same pitch and length as what you
+  heard
+- [ ] Change the project sample rate with ReaSampler 9000 loaded (and with it inactive/bypassed,
+  then re-enabled) — the sample still plays at correct pitch afterwards
+- [ ] Existing projects that used mismatched-rate samples WILL sound different now (correct pitch)
+  — reopen one and confirm it's the expected change
+- [ ] Same-rate samples sound exactly as before