feat(ring,api): pick destination clip then resolve matching chained morph

Tasks 3-6. Transition is now a directed clip-pair morph; ScaleRing.morph_for
looks up (from,to)->file. advance_ring returns structural steps (no file);
resolve_move picks each crossed altitude's clip THEN its morph, chained, and
reports the locked target. Fast spin keeps the whole chain (blended) instead
of collapsing. /api/ring/advance takes from_clip_id, draws picks, returns the
resolved chain. Dropped ring_move_to_dict + _rev_file.

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

simulator/app.py

@@ -29,11 +29,18 @@ from player.alteration import (
 from player.audio import AUDIO_SOURCES, VISUAL_POSITIONS, resolve_audio, resolve_visual
 from player.ring import (
     DEFAULT_FAST_SPIN_THRESHOLD,
+    ResolvedMove,
     advance_ring,
     pick_clip_id,
+    resolve_move,
     scale_at,
 )
-from simulator.clips import load_manifest, load_ring, ring_move_to_dict, ring_to_dict
+from simulator.clips import (
+    load_manifest,
+    load_ring,
+    resolved_move_to_dict,
+    ring_to_dict,
+)
 
 STATIC_DIR = Path(__file__).parent / "static"
 MEDIA_DIR = Path(__file__).parent / "sample_media"
@@ -87,12 +94,6 @@ def _media_version(rel: str) -> Optional[str]:
     return token
 
 
-def _rev_file(file: str) -> str:
-    """The baked zoom-out companion path for a transition file (mirrors the
-    client's `reverseFile`): `<edge>.mp4` -> `<edge>.rev.mp4`."""
-    return file[:-4] + ".rev.mp4" if file.endswith(".mp4") else file
-
-
 class ControlsModel(BaseModel):
     visual: str
     audio: str
@@ -119,6 +120,7 @@ class AlterationRequest(BaseModel):
 class RingAdvanceRequest(BaseModel):
     from_index: int = 0
     delta: int
+    from_clip_id: str = ""
 
 
 class AuthorTrackRequest(BaseModel):
@@ -213,14 +215,14 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
     @app.get("/api/media-versions")
     def api_media_versions():
         """Per-file content-hash tokens the client appends to /media URLs as
-        `?v=<hash>`. Covers every served file: each clip's base footage plus each
-        ring transition and its baked reverse. A re-baked clip's hash changes, so
-        its URL changes and the browser refetches — a permanent cache-bust."""
+        `?v=<hash>`. Covers every served file: each clip's base footage plus every
+        ring transition morph (both directions are explicit entries). A re-baked
+        clip's hash changes, so its URL changes and the browser refetches — a
+        permanent cache-bust."""
         files = {c.base_file for c in app.state.clips}
         if app.state.ring is not None:
             for t in app.state.ring.transitions:
                 files.add(t.file)
-                files.add(_rev_file(t.file))
         versions = {}
         for f in sorted(files):
             v = _media_version(f)
@@ -244,12 +246,20 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
             req.delta,
             fast_spin_threshold=DEFAULT_FAST_SPIN_THRESHOLD,
         )
-        # Rotating pool: pick a random member of the LANDED scale (content-pipeline
-        # §11.1). A delta=0 advance is the initial / re-roll pick (a no-op move that
-        # still yields a fresh random clip). The pure pick takes injected randomness.
-        landed = scale_at(app.state.ring, move.to_index)
-        chosen = pick_clip_id(landed, random.random())
-        return ring_move_to_dict(move, app.state.ring, chosen)
+        if move.steps:
+            # Choose the destination member of each crossed altitude FIRST (one
+            # random draw per step — content-pipeline §11.1), then resolve the
+            # morph from the prior clip to it, so the footage matches what we land
+            # on. The currently-shown clip threads in as `from_clip_id`.
+            src = req.from_clip_id or scale_at(app.state.ring, move.from_index).clip_id
+            picks = tuple(random.random() for _ in move.steps)
+            resolved = resolve_move(app.state.ring, move, src, picks)
+        else:
+            # A no-op move (delta=0) is the initial / re-roll pick: a fresh random
+            # member of the current scale, no morph.
+            landed = scale_at(app.state.ring, move.to_index)
+            resolved = ResolvedMove(steps=(), target_clip_id=pick_clip_id(landed, random.random()))
+        return resolved_move_to_dict(move, resolved)
 
     @app.post("/api/author/track")
     def api_author_track(req: AuthorTrackRequest):

simulator/clips.py

@@ -14,7 +14,7 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Any
 
-from player.ring import RingMove, Scale, ScaleRing, Transition, scale_at
+from player.ring import ResolvedMove, RingMove, Scale, ScaleRing, Transition
 
 
 @dataclass(frozen=True)
@@ -99,7 +99,12 @@ def load_ring(path: str | Path) -> ScaleRing | None:
         return None
     scales = tuple(_scale_from_dict(s) for s in ring.get("scales", []))
     transitions = tuple(
-        Transition(file=t["file"], model=t.get("model", ""))
+        Transition(
+            from_clip=t.get("from", ""),
+            to_clip=t.get("to", ""),
+            file=t["file"],
+            model=t.get("model", ""),
+        )
         for t in ring.get("transitions", [])
     )
     return ScaleRing(scales=scales, transitions=transitions)
@@ -127,35 +132,35 @@ def ring_to_dict(ring: ScaleRing, clips: list[Clip]) -> dict:
             }
             for s in ring.scales
         ],
-        "transitions": [{"file": t.file, "model": t.model} for t in ring.transitions],
+        "transitions": [
+            {"from": t.from_clip, "to": t.to_clip, "file": t.file, "model": t.model}
+            for t in ring.transitions
+        ],
     }
 
 
-def ring_move_to_dict(
-    move: RingMove, ring: ScaleRing, chosen_clip_id: str | None = None
-) -> dict:
-    """JSON form of an encoder move: the landing scale's clip and the ordered
-    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.
+def resolved_move_to_dict(move: RingMove, resolved: ResolvedMove) -> dict:
+    """JSON form of a RESOLVED encoder move: where it lands, the locked clip, and
+    the ordered chained morphs. Each step carries the clip it morphs FROM (the
+    clip currently shown), the chosen clip it morphs TO, and the matching morph
+    file (None if no morph was baked — the renderer plain-cuts). `fast` marks a
+    fast spin; each step then carries `blended` so the renderer accelerates it.
 
-    `target_clip_id` is the pool member the player should load on landing. The
-    caller passes the random pick (`pick_clip_id(landed_scale, random.random())`,
-    content-pipeline §11.1); when omitted it falls back to the scale's primary
-    `clip_id` (deterministic — keeps pre-pool callers/tests working)."""
+    `target_clip_id` is the final clip the viewer locks onto."""
     return {
         "from_index": move.from_index,
         "to_index": move.to_index,
         "wrapped": move.wrapped,
         "fast": move.fast,
-        "target_clip_id": chosen_clip_id or scale_at(ring, move.to_index).clip_id,
+        "target_clip_id": resolved.target_clip_id,
         "steps": [
             {
-                "edge": st.edge,
-                "reversed": st.reversed,
-                "file": st.file,
-                "to_index": st.to_index,
-                "blended": st.blended,
+                "from_clip": s.from_clip,
+                "to_clip": s.to_clip,
+                "file": s.file,
+                "to_index": s.to_index,
+                "blended": s.blended,
             }
-            for st in move.steps
+            for s in resolved.steps
         ],
     }