Phase 5: compositing depth ("C" complete)

Multi-layer stacks with blend modes, feedback, post chain, and a 3D
particle layer proving the compositor is genuinely hybrid. Looks now
generate accent layers from a different family, composited additively at
low opacity, weighted by section energy so intros stay sparse.

Adds flash-rate safety (engine/flash.js), which was not in the original
plan and should have been. This generates beat-reactive video for
publication, and rapid light-dark cycling is the photosensitive-epilepsy
trigger; WCAG 2.3.1 caps it at three flashes per second. Classic Wave
measured 7-8/s at every output resolution from 96x54 to 1920x1080, so it
was a real hazard rather than a sampling artefact.

Root cause was general, not one bad shader: `u_time * u_speed` where speed
is reactively modulated. Phase is elapsed*rate, so changing the rate at
time T jumps phase by T*delta — sixty seconds in, a 0.05 wobble throws the
phase three whole units between consecutive frames, and it worsens as the
track runs. Fixed by introducing rate params:

- schema flag `rate: true` documents and marks them
- Layer.resolveParams skips reactivity on them
- ArcDriver skips drift on them
- validateModule rejects a reactive entry on one
- lint-scenes greps shaders for `u_time * u_X` and fails if X is unmarked,
  so no future scene can reintroduce it

Every rate param across the six scenes is now marked. Two checks were
needed to find this: a per-look flash check, and a per-SCENE sweep at
aggressive params, since the look generator only samples part of the space
and a scene can hide an unsafe region for a long time.

Gate 10/10. Worst flash rate now 1/s. Feedback stable over 10,000 frames
(luminance 0.17-0.59, no saturation or decay). 0.17ms/frame at 1280x720.

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

flow-state/src/engine/Layer.js

@@ -53,6 +53,9 @@ export class Layer {
         for (const [name, r] of Object.entries(reactive)) {
             const def = defs[name];
             if (!def || def.type === 'palette') continue;
+            // Rate params multiply absolute time; modulating them jumps the phase
+            // by elapsed * delta. See params/schema.js RATE_FLAG.
+            if (def.rate) continue;
             const raw = features[r.feature];
             if (raw === undefined) continue;