docs: close out Θ-W2-T1 filter-voice-path into COMPLETED

Move the landed filter-voice-path track from PLAN.md into COMPLETED.md with full
narrative (quantizer removal, editor floor raise). Mark Θ-W2 fully landed, matching
W1's phrasing; fix a stale forward reference.

docs/COMPLETED.md

@@ -151,6 +151,44 @@ path.
   pre- vs post-envelope placement is sound-defining), the drive dial's calibration, and
   the fact that drive authority varies ~11 dB across the morph sweep.
 
+### Θ-W2-T1 — filter-voice-path
+
+Wires the pure filter module into ReaSampler 9000's per-voice signal path as a new fixed
+processing point between the pitch envelope and the amp stage, gives it its own
+knob-deck group, and relays the deck row in signal-flow order (pitch → filter → amp).
+Each `Voice` owns its own `VoiceFilter` and a second `AdsrEnvelope` instance — per-voice,
+never shared — and neither adds allocation or virtual dispatch to the per-sample path.
+Parameters — morph position, cutoff, Q, drive, mod amount (bipolar ±100%, targeting
+cutoff), velocity and key-tracking modulation, then AHDSR — live in the one parameter
+set; `FilterParams` stores the filter module's own `FilterSettings` by value rather than
+a parallel copy of the normalized positions. The filter is off by default and bit-exact
+off — a project saved before the change reopens sounding identical, pinned by a
+bit-equality test. `ComponentState`'s params payload moves v8 → v9, appending the filter
+tail; a v8 blob is a strict prefix and lifts to the off/neutral filter default, with
+non-finite filter fields falling back to neutral, pinned by a golden byte-literal
+fixture. Deck composition was extracted into a new pure module
+`core/instrument/ui/deck_groups`, with the pitch → filter → amp row order pinned by
+test; the filter's AHDSR ships as its own `FILTER ENV` group sibling to `FILTER`,
+mirroring the existing `PITCH` / `PITCH ENV` split, and a morph-law toggle (`Band` |
+`Notch`) ships as the FILTER group's row toggle.
+
+**Deviations from spec:**
+- **An initial mod-quantizer was added, then rejected and removed.** A
+  `kFilterModSteps = 2048` step gate on the coefficient re-solve stair-stepped the
+  corner (~5.8 cents per step); Daniel rejected it. Replaced by a cutoff-only re-solve
+  (`VoiceFilter::setCutoffNorm`) that re-derives only `g = tan(π·fc/sr)` — Q's parabola,
+  the morph's cos/sin, and the folded mix are all cutoff-independent and stay cached
+  from `prepare()`; `filter_params` hoists its constant logs. Measured at 48 kHz,
+  Release, net of the sweep generator: kernel alone 2.8 ns/frame, full `prepare()` 56.9
+  ns, cutoff-only re-solve 15.5 ns — about 1.2% of a core for 16 continuously-swept
+  voices. The corner now sweeps continuously rather than stair-stepping.
+- **The editor floor was raised to 840×620**, which is also its new default size, up
+  from a 560×460 floor at which the grown deck wrapped to four rows and pushed
+  `FILTER ENV` / `AMP ENVELOPE` / `VOICE` / `MASTER` off-screen with no scroll.
+  `kEditorMinWidth`/`kEditorMinHeight` now live in `sample_bands`, read by both the
+  shell's `checkSizeConstraint` and the opening `ViewRect`. Consequence worth
+  recording: a host with a saved editor rect below 840×620 is clamped up on reopen.
+
 ### Θ-W2-T2 — stereo-waveform-lanes
 
 Delivered as specified: two stacked lanes, L above R, in stereo mode; one lane in mono;

docs/PLAN.md

@@ -145,8 +145,8 @@ collision is real and the serialization is the correct answer.
 the full narrative of each. Between them: the zone subsystem is retired, the wave's two
 responsibility seams (the `sampler_core` split, the Sample-face band split) are in
 place, the two extension-side capture-handoff defects are fixed, and the filter DSP has
-landed as a standalone pure module with no call site — Θ-W2-T1 integrates it into the
-voice path.
+landed as a standalone pure module — Θ-W2-T1 has since wired it into the voice path (see
+below).
 
 ---
 
@@ -158,77 +158,14 @@ band-stack allocator that T2 and T3 fill; and W1-T1's key-range answer, which de
 what T3's piano strip displays. Authoring any of this against the per-zone model means
 writing storage plumbing W1 deletes.
 
-**Θ-W2-T2 (`stereo-waveform-lanes`) and Θ-W2-T3 (`toolbar-and-piano-strip`) have
-landed** — see `docs/COMPLETED.md`. Between them: the waveform band now shows both
-channels in stereo mode behind a type-enforced full-height overlay contract, and the
-chrome band's toolbar and piano strip are cleaned up per spec. **One track remains:**
-Θ-W2-T1 (`filter-voice-path`), which puts the filter into the per-voice signal path and
-gives it its deck. The three tracks were disjoint by band — T1 owns the parameter model
-and the deck band, T2 owned the waveform band, T3 the chrome band — and none
-re-allocates the band stack.
-
-#### Θ-W2-T1 — `filter-voice-path`
-
-**Goal.** Put the filter into the per-voice signal path as a new fixed processing point,
-give it its deck, and relay the deck row in signal-flow order.
-
-**Consolidates item 2 (integration half).** The DSP module is Θ-W1-T3. The filter
-envelope's *curve treatment and mode-driven shape* arrive in Θ-W3 — this track ships the
-filter envelope in the existing staged AHDSR shape.
-
-**Surface boundary — owns:** `core/instrument/engine/` voice-render filter call site +
-`zone_params.h` (filter parameters), `core/instrument/map/component_state_io` (version
-bump for the filter parameters), `core/instrument/ui/knob_deck` + the **deck band** of
-the Sample face (geometry, paint, input). Reads the band allocator; does not change it.
-
-**Behavior.**
-- **Pipeline position.** A new processing point in the sampler audio pipeline: **after
-  the pitch envelope, before the amp stage.** The amp envelope still shapes the filtered
-  result.
-- **Per-voice.** Each sounding voice runs its own filter with its own envelope state —
-  not a shared instance-wide filter.
-- **Parameters:** mode, cutoff, Q, mod amt — then the AHDSR controls.
-  - **Mod amt: bipolar, −100% to +100%, targeting cutoff** — covering the full range
-    from either end.
-  - Cutoff / Q / mode ranges are the module's, from Θ-W1-T3.
-- **Velocity and key-tracking modulation ship with this item — not deferred.** The
-  filter gains velocity modulation following the amp's velocity-transfer-curve pattern
-  and key-tracking following the pitch key-tracking pattern. The follow-up established
-  the parallel, not new ranges or control layout — follow the cited precedents. (The
-  *bipolar domain and default* of the filter velocity curve, and its button's home, are
-  item 11 — Θ-W4-T2. This track wires the modulation path; that track shapes the curve.)
-- **Off by default.** Pre-existing saved instances and freshly loaded captures sound
-  unchanged until the user engages it.
-- **Parameters live in the instrument's one parameter set** (post-item-16) — no per-zone
-  storage, no Sample/Zone parity.
-- **Label.** The user-facing deck-group label is **"Filter"**. "MM preamp" is working
-  shorthand for the DSP lineage, not UI text.
-- **Deck reorder.** The knob deck row is relaid out in signal-flow order:
-  **pitch → filter → amp**.
-
-**Acceptance criteria.**
-- With the filter engaged, played notes are audibly filtered at the specified pipeline
-  point: the filter acts on pitched (post-pitch-envelope) signal and the amp envelope
-  still shapes the filtered result — audible ordering pitch → filter → amp.
-- Mode, cutoff, Q, and mod-amt controls appear in a deck group labeled **"Filter"**.
-- Cutoff sweeps the full audio spectrum on a log scale, fully open to fully closed; Q
-  spans 0.1 → 10 with √2 at the control's center; mod amt at −100% and at +100% each
-  drive cutoff across the full range, from opposite ends.
-- High Q audibly emphasizes the cutoff region in both HP and LP modes.
-- **Two simultaneously sounding voices at different envelope phases are filtered
-  independently** — per-voice processing is audible, not a shared filter.
-- Filter parameters live in the one parameter set: they edit in one place and govern the
-  instrument as a whole.
-- Velocity and key-tracking modulation of the filter are audible — velocity following
-  the amp-velocity-transfer-curve pattern, key-tracking following the
-  pitch-key-tracking pattern.
-- The deck row reads pitch → filter → amp left-to-right.
-- **The filter is off by default:** a project saved before this change reopens sounding
-  identical, and a freshly loaded capture sounds unchanged until the filter is engaged.
-- The filter tick adds no allocation and no virtual dispatch to `process()`.
-
-**Open questions.** None carried from item 2 — all three of its prior questions (other
-mod sources, parameter storage side, neutral default) closed in the source doc.
+**All three tracks have landed** — Θ-W2-T1 (`filter-voice-path`), Θ-W2-T2
+(`stereo-waveform-lanes`), and Θ-W2-T3 (`toolbar-and-piano-strip`) — see
+`docs/COMPLETED.md` for the full narrative of each. Between them: the filter sits in the
+per-voice signal path with its own deck, the waveform band shows both channels in
+stereo mode behind a type-enforced full-height overlay contract, and the chrome band's
+toolbar and piano strip are cleaned up per spec. The three tracks were disjoint by
+band — T1 owned the parameter model and the deck band, T2 owned the waveform band, T3
+the chrome band — and none re-allocated the band stack.
 
 ---