feat(ring): fast-spin blended pass past a speed threshold (scales design §3)

A multi-detent encoder spin previously chained one full transition per scale
crossed (e.g. +5 ≈ 5 placeholder morphs ≈ 12-15s), which feels sluggish for a
fast spin. Design §3 anticipates this: "transitions chain, or past a speed
threshold a faster blended pass is used."

`player.ring.advance_ring` gains an opt-in `fast_spin_threshold` (canonical
default `DEFAULT_FAST_SPIN_THRESHOLD = 3`). A spin batches its detents into one
advance() call, so `abs(delta)` is the input layer's proxy for spin speed; at or
above the threshold the move collapses to a single blended pass — one
`TransitionStep` (the arrival edge, marked `blended`, landing straight on the
destination, `RingMove.fast=True`) instead of the full chain. Landing index,
seam-crossing (`wrapped`), and arrival edge/direction are exactly the full
chain's; only the in-between transitions are dropped.

The policy is opt-in (default off) because the pure function cannot observe
wall-clock spin speed — the simulator/firmware, which can, enables it. This keeps
all existing complete-chain behavior and tests intact. The simulator passes the
default threshold and plays the blended step at 2.5× (FAST_BLEND_RATE); a
dedicated fast-blend clip can replace the accelerated arrival-edge placeholder
when real transition media exists.

- player/ring.py: TransitionStep.blended, RingMove.fast, threshold param + policy
- simulator/clips.py: serialize fast/blended in ring_move_to_dict
- simulator/app.py: apply DEFAULT_FAST_SPIN_THRESHOLD at /api/ring/advance
- simulator/static/app.js: play a blended step at FAST_BLEND_RATE
- docs: scales design §3, USER_GUIDE, ROADMAP slice-3 note

Tests: +9 (ring policy, serializer, API). Suite 224 passed / 2 skipped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/static/app.js

@@ -126,9 +126,13 @@ function debounced() { clearTimeout(timer); timer = setTimeout(update, 80); }
 
 // --- Scale ring (endless encoder) ---
 
-function playTransition(file) {
-  // Play one placeholder zoom/warp transition clip start-to-finish, with the
-  // alteration layers hidden. Resolves on 'ended' (or a safety timeout).
+const FAST_BLEND_RATE = 2.5;   // playback speed for a collapsed fast-spin pass
+
+function playTransition(file, blended) {
+  // Play one zoom/warp transition clip start-to-finish, with the alteration
+  // layers hidden. A `blended` (fast-spin) pass runs at FAST_BLEND_RATE so a
+  // quick encoder spin lands fast instead of grinding through every morph.
+  // Resolves on 'ended' (or a safety timeout that scales with playback rate).
   return new Promise((resolve) => {
     overlay.style.opacity = "0";
     tint.style.opacity = "0";
@@ -136,11 +140,18 @@ function playTransition(file) {
     vid.loop = false;
     vid.style.opacity = "1";
     vid.src = mediaUrl(file);
+    vid.playbackRate = blended ? FAST_BLEND_RATE : 1;
     let done = false;
-    const finish = () => { if (done) return; done = true; vid.removeEventListener("ended", finish); resolve(); };
+    const finish = () => {
+      if (done) return;
+      done = true;
+      vid.removeEventListener("ended", finish);
+      vid.playbackRate = 1;
+      resolve();
+    };
     vid.addEventListener("ended", finish);
     vid.play().catch(finish);
-    setTimeout(finish, 6000);
+    setTimeout(finish, blended ? 3000 : 6000);
   });
 }
 
@@ -154,8 +165,10 @@ async function advance(delta) {
     });
     if (!resp.ok) return;
     const move = await resp.json();
+    // A fast spin comes back collapsed to a single blended step (move.fast);
+    // a slow spin chains one full transition per scale crossed.
     for (const step of move.steps) {
-      await playTransition(step.file);
+      await playTransition(step.file, step.blended);
     }
     ringIndex = move.to_index;
     currentVariant = -1;        // force the target clip's variant to (re)load