docs: close out wave Theta-W4, and defer the deck layout rework to TODO

docs/COMPLETED.md

@@ -460,3 +460,47 @@ bumped to **v11** (`kParamsLoopVersion`), appended at the tail; slot 12 is reser
 smooth on real material, whether the top-strip tab is discoverable, and the LICE
 rendering of the tab and crossfade fill. Also open: whether the crossfade default
 should stay 0 (a smooth seam becomes opt-in).
+
+### Θ-W4-T2 — velocity-deck-and-bipolar-curves
+
+Gives the three velocity-curve popups (amp, pitch, filter) one home — a new deck group
+labelled VELOCITY — and makes the pitch and filter transfer curves bipolar. No
+velocity-curve button remains in MASTER, PITCH, or Filter. Pitch and filter curves now
+run y range [−1, 1], default flat at 0, so velocity modulation of pitch and filter is off
+until the user draws a curve; amp stays unipolar [0, 1] with its flat-unity default
+unchanged. The domain is modelled as a `CurveDomain { Unipolar, Bipolar }` field on
+`VelocityCurve`, with `curveYMin`/`curveNeutral` deriving from it; `VelocityPoint::amp`
+was renamed to `value`. A velocity→pitch transfer curve is new — it did not previously
+exist. Full scale is `kVelocityPitchRangeSemitones = 24.0`, now the single constant the
+shell's pitch-depth control also consumes; it folds into `baseRatio_` once at note-on, so
+`process()` gains no per-frame work. The preview button's text is replaced by a drawn
+play triangle — `previewGlyph()` returns three vertices from the pure layer, the shell
+passes them to `LICE_FillTriangle`, which was already in the build: no new dependency, no
+asset. Params payload is **v12** (`kParamsVelocityVersion = 12`), appending the
+velocity→pitch curve after Θ-W4-T1's loop block.
+
+**Daniel's ruling — the depth knob stays.** The implementation initially *removed*
+`FilterParams::velAmount` and the `kFilterVel` depth knob, arguing a bipolar curve is
+both shape and amount. Daniel rejected that: the knob scalar AND the curve both apply.
+The depth control was restored, and the filter's velocity contribution is
+`velAmount × curve.eval(v)` with the curve bipolar. Consequence: with `velAmount`
+surviving, the pre-v12 migration became a **pure domain re-tag** — a pre-v12 unipolar
+curve's y values already sit inside [−1, +1], so `velAmount` and every knot carry
+forward bit-identically, with no scaling transform and no version branch in the reader.
+The earlier fold-and-rescale approach (and its degree-1-homogeneity argument, which was
+only exact to within double rounding) was removed entirely.
+
+**`kFilterVel` also crossed from non-live to live** — a user-visible contract change
+beyond simple restoration. Rationale: it is a depth over a latched value, the same shape
+as `kFilterKeyTrack`, live since Θ-W3; the note latches `curve.eval(velocity)` and the
+depth multiply happens in `applyLive` at block boundaries, gliding through the existing
+cutoff ramp at zero per-sample cost.
+
+**Deviations from spec / code review:** Code review ran on two surfaces
+(engine/persistence, UI/editor) and found one Critical plus two actionable Majors and ten
+Minors, all remediated. The Critical: `editedCurve()`'s `kNone` fallback let
+Esc-during-a-curve-node-drag write the pitch or filter curve — bipolar domain and all —
+over the amp gain curve and persist it. Fixed on both routes (the popup close now
+cancels the drag; the mutable accessor refuses `kNone`). It has **no automated
+regression pin** — `src/shell/instrument/` has no test target, and the bug is shell
+state-machine coupling with no pure-layer equivalent.