docs: close out Theta-W4-T1 into COMPLETED, collapse the T1 spec, and record the frames-not-ms crossfade ruling

docs/PLAN.md

@@ -203,57 +203,24 @@ button in the chrome band. Neither touches the other's band.
 
 #### Θ-W4-T1 — `gate-loop-sustain`
 
-**Goal.** Establish (or restore) loop points, and make a Gate-mode loop function as the
-sustain — indefinite playback until note-off, with a crossfaded seam.
-
-**Consolidates item 9 (both halves: the suspected regression and the feature spec).**
-
-**Surface boundary — owns:** the voice-render loop read path (post-W1 seam),
-`core/instrument/map/component_state_io` (loop parameters), `core/instrument/ui/
-waveform_view` markers, and the **waveform band's marker layer**. Reads W2-T2's lane
-geometry and W3's overlay contract; re-owns neither.
-
-**Behavior.**
-- **Regression half.** Daniel's observation: loop points appear to have been lost.
-  Whether they were genuinely removed from playback or are merely unexposed in the
-  current UI **must be verified as the first act of implementation** — it was not
-  guessable in the source doc, which was written under a no-code-reads constraint.
-- **Feature half.** In **Gate mode**: a loopable section with **parameterized start and
-  end points** and a **parameterized crossfade on loop reset**. The loop functions as the
-  sustain — indefinite playback cycling the loop until note-off, then release.
-- **Loop-sustain is a Staged/Gate-mode feature.** Gate mode is unavailable while a
-  Spline EG is active (item 3, Θ-W5), so Spline mode remains full-sample-length playback
-  with no loop. This track must not assume Gate is always available.
-- **Storage:** loop parameters belong to the instrument's one parameter set. There is no
-  per-zone side to confirm.
-- **Preserve-mode contract (existing, do not break):** *loop the source, shift the
-  output* — loop points stay source-frame facts.
-- **Reset-scope note for Ξ:** item 15 classifies loop points as **baked-in** — they
-  shaped the render and old positions are meaningless against new audio — so they reset
-  on a resample. Nothing to build here; recorded so Ξ-W2-T1 does not re-derive it.
-
-**Acceptance criteria.**
-- In Gate mode with a loop defined, a held note sustains indefinitely, audibly cycling
-  the loop section; note-off exits into the release stage.
-- With a nonzero crossfade the loop seam is smooth — **no click at the loop reset** — and
-  crossfade length audibly follows its parameter.
-- Loop start, end, and crossfade are user-parameterized, editable, and persisted across
-  save/reload.
-- Whatever the regression finding, the end state is loop points exposed and functional
-  per this spec.
-- Looping adds no allocation to `process()`; the crossfade is computed on the per-sample
-  path without a branch-heavy or virtual detour.
-
-**Open questions.**
-- **Regression verification — removed vs. unexposed [verify].** The first act of
-  implementation. Report the finding; the end state is the same either way.
-- **Crossfade units and range [propose].** Unspecified. Surface a proposed default at
-  implementation review (ms is the likely denomination given item 10 puts time constants
-  in ms).
-- **Editing surface for loop start/end [propose].** Waveform markers, knobs, or both, is
-  unspecified. The waveform display is the natural home for range markers — and
-  `waveform_view` already owns generic named draggable markers with clamp and
-  zero-crossing snap, which is a strong lean — but Daniel has not said. Propose at review.
+**Has landed** — see `docs/COMPLETED.md` for the full narrative. Loop points are now a
+usable feature and a Gate-mode loop functions as the sustain. The regression half
+resolved as present-but-unreachable, not removed: nothing in any capture path ever wrote
+`Sample::loop`, so every capture opened with `hasLoop == false`, and the ghost default
+parked `loopStart` at frame 0 directly under the start marker, where `markerAtPoint`'s
+first-in-draw-order tie-break made the handle ungrabbable. Fixed by moving the ghost
+span to `defaultLoopBounds` (last quarter, both handles clear), making a collapsed span
+the explicit OFF gesture, and adding a parameterized crossfade — stored in source
+frames, not ms (a deliberate divergence from the plan's ms lean, following
+`sample_map.h`'s source-frame rule), linear rather than equal-power, and normalized over
+`crossfade − 1` (a review Major fixed a residual seam discontinuity) so the last
+rendered frame lands exactly on the incoming tap. New pure module
+`src/core/instrument/engine/loop/` (`loop_span`) holds the loop math; the editing
+surface is the waveform markers plus a new top-strip `markerHandleRect` grab tab, a
+general fix for markers sharing a frame. Params payload is v11 (`kParamsLoopVersion`);
+slot 12 is reserved for T2. Left to Daniel: whether the seam sounds smooth on real
+material, the top-strip tab's discoverability, the LICE rendering of the tab and
+crossfade fill, and whether the crossfade default should stay 0.
 
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