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/scenes/shader/firefly-drift.js

@@ -14,7 +14,7 @@ export const fireflyDrift = {
     traits: ['camera', 'style'],
 
     params: {
-        count:    { type: 'float', range: [6, 48],   default: 22,   uniform: 'u_count', bias: 'density'  },
+        count:    { type: 'float', range: [6, 48],   default: 22,   uniform: 'u_count', bias: 'density', discrete: true  },
         speed:    { type: 'float', range: [0.05, 0.8], default: 0.3, uniform: 'u_speed', bias: 'motion', rate: true },
         jitter:   { type: 'float', range: [0, 0.4],  default: 0.12, uniform: 'u_jitter' },
         spread:   { type: 'float', range: [0.4, 1.3], default: 0.95, uniform: 'u_spread' },