Stack L/R waveform lanes in stereo mode, with overlays drawn once at full band height

src/core/instrument/CLAUDE.md

@@ -220,7 +220,8 @@ slider couldn't. Two pure modules split the forward (draw) and inverse (edit) ma
 - `sample_bands` — **THE band-stack allocator**, and the only module that owns the Sample face's vertical inventory: three bands top-to-bottom (CHROME toolbar+control row / WAVEFORM elastic, floored at two stacked lanes / DECKS bottom-anchored at the knob deck's own wrapped height), plus the waveform band's lane split. A shared READ-ONLY surface for every band owner — a band's interior module lays out inside the rect it is handed and never re-allocates the stack.
 - `sample_chrome` — the CHROME band's interior: the toolbar row (title + Browse) over the control row (root strip, preview, velocity knob cell, curve button, channel toggle). The fixed run is right-anchored; the root strip takes the remainder.
 - `keyboard_strip` — piano-keyboard strip: MIDI-note→key rect mapping, black/white key layout, hit-test, root-marker rect, and the drag-delta note resolver.
-- `waveform_view` — waveform/marker geometry: maps frame span linearly across a rect; generic named draggable markers with drag-delta resolver, clamp, and zero-crossing snap.
+- `waveform_view` — the WAVEFORM band's interior: `waveformSurface` resolves the drawn lane(s) (two stacked lanes, L over R, only when the mode is stereo AND the source has a second channel — a mono source under stereo mode is dual-mono and draws one lane) plus **the** overlay rect, and `laneEnvelope` splits one multi-channel envelope pass per lane. Also maps frame span linearly across a rect; generic named draggable markers with drag-delta resolver, clamp, and zero-crossing snap.
+  - **Overlay contract (consumed by later waveform work).** `WaveformSurface::overlay` — equivalently the standalone `waveformOverlayArea(band)` — is the FULL band in both modes. Everything riding the waveform (the amp-envelope trace and its node handles, the start/loop markers, the loop region) draws ONCE into it, spanning both stacked lanes; hit-testing resolves against the same rect so a grab in the lower lane reaches them. Anything drawn or hit-tested per lane is a duplicate and a defect.
 - `capture_browser` — capture browser: card-grid layout + bank-filter tab strip geometry and hit-test; knows only counts and rects, draws nothing.
 - `browser_scroll` — scroll + type-to-filter layered over `capture_browser`: vertical scroll offset, scrollbar thumb, thumb-drag mapping, and name-substring search.
 - `param_slider` — parameter control-panel: vertical stack of TOGGLE (two-segment selector) and SLIDER (horizontal track) rows; maps normalized value to/from handle pixel.