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/app.py

@@ -23,7 +23,7 @@ from player.alteration import (
 )
 from player.content import resolve_content
 from player.controls import CONTENT_POSITIONS
-from player.ring import advance_ring
+from player.ring import DEFAULT_FAST_SPIN_THRESHOLD, advance_ring
 from simulator.clips import load_manifest, load_ring, ring_move_to_dict, ring_to_dict
 
 STATIC_DIR = Path(__file__).parent / "static"
@@ -112,7 +112,12 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
     def api_ring_advance(req: RingAdvanceRequest):
         if app.state.ring is None:
             raise HTTPException(status_code=404, detail="no scale ring in manifest")
-        move = advance_ring(app.state.ring, req.from_index, req.delta)
+        move = advance_ring(
+            app.state.ring,
+            req.from_index,
+            req.delta,
+            fast_spin_threshold=DEFAULT_FAST_SPIN_THRESHOLD,
+        )
         return ring_move_to_dict(move, app.state.ring)
 
     if MEDIA_DIR.exists():