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/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.
 
 ---