Filesystem routes, sections, dynamic segments; instant YES hero

The questions/ tree is the router now: ids derive from file paths
(index.yaml names its directory; explicit id still wins for legacy
content), actions and requires_chain accept relative refs, nested
non-index files infer followup, and _section.yaml applies qualifies/
requires_chain/responsible to everything under its directory. Dynamic
[name].yaml pages serve any /dir/<value> with the segment substituted
into {name} resource-key placeholders; submissions index their chain
node in a portal_chains KV so requires_chain pages can verify a
visitor's ?chain= lineage actually ends at the required question.
Loading uses one recursive git-trees call; question_lint walks
subdirectories the same way. Implements docs/design/filesystem-routes.md.

Also: the YES hero now starts at HTML parse time via an inline module
script (yes.js moved to public/ for a stable /yes.js the wasm binding
raw_module-imports too - snippet paths are per-build-hashed), with
hydration adopting the running instance; and both gesture containers
reserve their box in CSS so mounting doesn't shift content.

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

style/main.css

@@ -270,6 +270,11 @@ main.not-found {
   position: relative;
   width: 100%;
   max-width: 46rem;
+  /* Same jump-avoidance as .gesture-wrap: the canvas (55svh tall)
+     only exists after hydration, so hold its height open from the
+     first paint. Plain-vh fallback first, like the canvas itself. */
+  min-height: 55vh;
+  min-height: 55svh;
 }
 
 .hero-gesture .gesture-canvas {
@@ -559,6 +564,11 @@ textarea:focus {
    custom properties - so palette changes go there AND here. */
 .gesture-wrap {
   position: relative;
+  /* The canvas is created by JS only after wasm hydration - reserve
+     its 3/2 box now so content below doesn't jump when it appears.
+     aspect-ratio is a preferred size, so the box still grows when the
+     echo thumbnail strip shows up under the canvas. */
+  aspect-ratio: 3 / 2;
 }
 
 .gesture-canvas {