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/clips.py

@@ -107,11 +107,13 @@ def ring_to_dict(ring: ScaleRing, clips: list[Clip]) -> dict:
 
 def ring_move_to_dict(move: RingMove, ring: ScaleRing) -> dict:
     """JSON form of an encoder move: the landing scale's clip and the ordered
-    transition clips to play (with direction)."""
+    transition clips to play (with direction). `fast` flags a collapsed fast-spin
+    pass; the single step then carries `blended` so the renderer plays it quick."""
     return {
         "from_index": move.from_index,
         "to_index": move.to_index,
         "wrapped": move.wrapped,
+        "fast": move.fast,
         "target_clip_id": scale_at(ring, move.to_index).clip_id,
         "steps": [
             {
@@ -119,6 +121,7 @@ def ring_move_to_dict(move: RingMove, ring: ScaleRing) -> dict:
                 "reversed": st.reversed,
                 "file": st.file,
                 "to_index": st.to_index,
+                "blended": st.blended,
             }
             for st in move.steps
         ],