Phase 4: arc driver ("C" brain)

Three timescales now stack: per-frame reactivity, per-section seeded LFO
drift, and whole-song scene changes with lookahead. Layer instances are
cached per section and reused across crossfades — rebuilding them per frame
would recompile shaders every transition.

Crossfades run forward from a boundary: the outgoing scene holds while the
incoming one fades in over it.

Three real bugs, each found by a check that had to be rewritten first:

1. A pop exactly at every transition. buildSlope is discontinuous by
   construction (~1 before a boundary, 0 after), and the outgoing layer is
   still on screen when it flips — collapsing its lookahead ramp in one
   frame. It now holds the slope it had entering the boundary.

2. FeatureTrack.at() returns a REUSED row object, and _boundarySlope()
   called at() again mid-render, rewriting the features the layer was about
   to read. Symptom: a frame correct on every repeat and wrong the first
   time — invisible to fresh-vs-fresh comparison, and wrong in every export,
   since export renders each frame exactly once. Now indexes the typed array
   directly, with the aliasing hazard documented on at(), and a new check
   covers the whole bug class.

3. Warm-up converged to 1%, leaving a visible 0.015 difference at heavy
   feedback settings. Now targets 0.1%.

Two checks were themselves wrong and were rebuilt: a raw delta threshold
and an outlier-vs-local-median test both flag beat flashes as pops, and a
control window taken from a different scene reads an ordinary busy scene as
a 9x spike. The working formulation A/Bs each boundary against the interior
of the two scenes adjacent to it.

PLAN.md §6 corrected: boundary seeks are NOT exact for free. Layer state is
re-seeded there but the feedback buffer is global and carries across.
Clearing it at boundaries would buy exactness for a visible flash at every
transition; warm-up is the better trade and applies everywhere.

Gate 9/9.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flow-state/PLAN.md

@@ -266,15 +266,24 @@ Stateful layers — feedback buffers, particle systems — mean frame *N* depend
 before it. Scrubbing to an arbitrary frame therefore can't be exact for free. Three-part
 resolution:
 
-1. **Warm-up.** On seek, render *K* frames (default ≈120, about 2s) as fast as possible
-   off-screen before displaying. Converges feedback state to visually correct.
-2. **Section-boundary seeks are exact.** Particle systems and layer state are re-seeded
-   deterministically at each section boundary, so jumping to a boundary — the common review
-   action — needs no warm-up and is frame-exact.
-3. **Export is always sequential**, so it is exact everywhere by construction.
+1. **Warm-up.** On seek, render *K* frames off-screen before displaying. *K* is computed
+   from the feedback decay rather than fixed — the residual after *n* frames is `decay^n`,
+   so `log(0.001)/log(decay)` frames converges to a tenth of a percent (96 frames at
+   decay 0.93). A light-feedback look seeks almost instantly; a heavy one still lands.
+2. **Export is always sequential**, so it is exact everywhere by construction.
 
-Documented consequence: scrubbing to a random mid-section frame shows a *converged*, not
-bit-exact, image. Reviewing a transition never hits this, because transitions are boundaries.
+> **Correction, made during Phase 4.** The original plan claimed section-boundary seeks
+> would be frame-exact with no warm-up, because layer state is re-seeded there. That is
+> wrong: layer state is only half the story, and the compositor's feedback buffer is
+> *global* — it carries straight across a boundary like any other frame. Making boundary
+> seeks exact would mean clearing feedback at every transition, which trades a cheap
+> warm-up for a visible flash on every scene change. Warm-up is the better trade, and it
+> applies everywhere rather than only mid-section. Measured: converges to a 0.00000 mean
+> difference. With feedback disabled, seeks are bit-exact anywhere, which is what proves
+> nothing *else* in the pipeline is carrying state.
+
+Documented consequence: scrubbing shows a *converged*, not bit-exact, image whenever
+feedback is enabled — which is visually indistinguishable, and exact once feedback is off.
 
 ---