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/checks/phase11.js

@@ -21,7 +21,7 @@ import { shiftPalette, paletteContrast } from '../look/palette.js';
 import { ArcDriver } from '../look/ArcDriver.js';
 import { Engine } from '../engine/Engine.js';
 import { featureProviderFor } from '../audio/FeatureTrack.js';
-import { sampleValues, clampValue, canBackground } from '../params/schema.js';
+import { sampleValues, clampValue, canBackground, isDiscreteParam } from '../params/schema.js';
 import { Rng } from '../engine/rng.js';
 import { frameDistance, frameLuminance } from '../engine/hash.js';
 import { SHOT_SIZES, SHOT_SIZE_NAMES, describeFraming } from '../look/framing.js';
@@ -505,6 +505,23 @@ check(11, 'the slow axis is a journey rather than a cycle', () => {
         const end = at(t.duration * 0.95);
 
         for (const item of axis) {
+            // Discrete params step at cuts instead of travelling: held within
+            // the shot by construction (see schema.js isDiscreteParam), so the
+            // travel assertion below cannot apply. What is asserted instead is
+            // the hold itself — two samples inside one cue must agree exactly —
+            // while the travel magnitude is already covered structurally above.
+            if (isDiscreteParam(item.def)) {
+                const t0 = (cue.startFrame + 5) / 60;
+                const t1 = (cue.endFrame - 5) / 60;
+                if (t1 > t0) {
+                    const va = at(t0)[item.name], vb = at(t1)[item.name];
+                    if (va !== vb) {
+                        problems.push(`${spec.module.name}.${item.name}: ${va} → ${vb} ` +
+                            `mid-shot — discrete params change only at cuts`);
+                    }
+                }
+                continue;
+            }
             const [lo, hi] = item.def.range;
             const span = hi - lo;
             const a = start[item.name], m = mid[item.name], z = end[item.name];