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/venues/ride.js

@@ -764,7 +764,7 @@ export const ride = {
         // whole town; light/fog still arc within each.
         if (instance.roadside) {
             const n = instance.roadside.children.length;
-            const dense = (arrangement % 2 === 1) || (vState && vState.popMul > 0.8);
+            const dense = (arrangement % 2 === 1);
             const showUpTo = dense ? n : Math.max(3, Math.floor(n * 0.45));
             // A billboard shot with a hidden billboard is an empty road: the
             // board's own slot is always on while it is the subject.