Continuity rule: discrete params hold per cue; add DESIGN.md + AGENTS.md

Actors and objects change only at camera cuts. int/bool/discrete
params skip drift, lookahead and reactivity; discrete slowAxis steps
on the section-held journey. Venues fade instead of toggling on
interpolated popMul; roadside density is arrangement-only.

DESIGN.md collects the project's design decisions; AGENTS.md points
agents at it and the build-visualizer loop.

flow-state/src/engine/Layer.js

@@ -4,7 +4,7 @@ import {
     SIGNATURE_UNIFORMS, IDENTITY_UNIFORMS, IDENTITY_ARRAY_UNIFORMS,
 } from './shader-contract.js';
 import { signatureUniforms, NEUTRAL_UNIFORMS } from '../look/Personality.js';
-import { clampValue } from '../params/schema.js';
+import { clampValue, isDiscreteParam } from '../params/schema.js';
 import { surfaceOf } from '../scenes/surface.js';
 import {
     CANVAS_BEAT_CAP,
@@ -208,6 +208,9 @@ export class Layer {
             // Rate params multiply absolute time; modulating them jumps the phase
             // by elapsed * delta. See params/schema.js RATE_FLAG.
             if (def.rate) continue;
+            // Continuity rule (see params/schema.js isDiscreteParam): a reactive
+            // count pops members on every beat. Discrete params hold per cue.
+            if (isDiscreteParam(def)) continue;
             const raw = features[r.feature];
             if (raw === undefined) continue;