Model head anchors and independent pose timing

docs/animation-model.md

@@ -339,54 +339,47 @@ head is placed and scaled where it belongs and the rest of the frame is simply
 not on stage. The full frame stays *available* for tracing without being
 *visible*, and those are different requirements.
 
-## The anchor: stabilisation is a channel, not a mode
+## Head motion: free or anchored to measured frames
 
-`stabilize` produces `{s, θ, tx, ty}` per frame, which is exactly
-`[:xform :scale]`, `[:xform :rot]` and `[:xform :pos]`. So removing the head's
-motion is not a pipeline setting — it is a question of **which node holds that
-motion**, and the answer is one channel definition:
+`stabilize` produces `{s, θ, tx, ty}` per source frame. Its inverse is stored
+densely on `:head`'s position, rotation and scale channels. The same measured
+track serves every placement choice:
 
 ```clojure
-;; locked: the head sits still, for tracing and for judging articulation
-[:xform :pos] {:animated? false :value [0.0 0.0]}
-
-;; as filmed: the head moves around the stage
-[:xform :pos] {:animated? true :interp :hold
-               :dense {:store "sha256:…" :stride 2 :frames 600}
-               :generated {:by :anchor/similarity}}
-
-;; per plate: the head snaps at each selected frame and holds
-[:xform :pos] {:animated? true :interp :hold :keys {0 […], 12 […], 23 […]}}
+;; no :anchors — free: read the measured transform at the current frame
+;; one key — lock to a chosen measured frame throughout
+:anchors {0 12}
+;; several keys — cut to another measured head transform at frame 40
+:anchors {0 12, 40 42}
 ```
 
-The three modes are the three channel shapes, on one channel, on one node. The
-third is the one a plate strip wants — the head pose is stable for exactly as
-long as a drawing is on screen — and it costs nothing because `:keys` already
-exists. Its frame set is the kept-frame set, which is `suggestPlateFrames` in the
-prototype and belongs to painting rather than to measurement.
+The map is `local change frame -> measured source frame`. A single lock is a
+one-key map. Position, rotation and scale read the same held source frame. The
+frame set belongs to head placement, independently of plate drawings and stage
+pose cuts. No measured block is copied into authored transform keys.
 
-**Always measure, always store factored, toggle the parent.** The fit is computed
-and the geometry is stored head-local in every mode, and only the parent's
-channel changes. Two things downstream require it, and both would be lost by
-making this an analysis-time switch:
+**Always measure, always store factored.** The fit is computed and the geometry
+is stored head-local in every mode. Only the frame address used to read the
+head's measured transform changes. Two things downstream require that split:
 
 - *Smoothing.* "Smooth the transform, never the contour" only means anything
   while the two are separate.
 - *Key selection.* A velocity minimum is "articulation paused" in head-local
   space and "the head happened to be still" in image space.
 
-It also makes the toggle an edit to the document rather than a reason to
-re-analyse: tier 1, undoable, syncable, and instant.
+This is a document edit, not a reason to re-analyse. A registered tracing photo
+will use its own source frame's stabilising transform followed by the same
+selected head placement, so it aligns with the vectors drawn over it.
 
 ### Two nodes, because two different things want that transform
 
 ```
 :face      group — AUTHORED.  where the face sits on the stage, and how big.
-  :head    group — MEASURED.  the head's motion, or identity.
+  :head    group — MEASURED.  dense head motion read at the selected frame.
     :mouth :mouth-in :teeth :lid-r :lid-l :brow-r :brow-l …
 ```
 
-Switching modes rewrites `:head` and never touches `:face`, so it cannot move
+Changing anchor keys edits `:head` and never touches `:face`, so it cannot move
 something that was placed by hand. A group node is free, and keeping the authored
 and the measured transform apart is the whole reason the transform is decomposed
 in the first place.
@@ -627,12 +620,12 @@ Proof that it covers what exists, not just what is wanted:
 | brow ring + quantised raise | node `:brow-r`, `[:geom :pts]` dense (the traced ring with height removed), `[:xform :pos]` dense (the quantised raise). **The decomposition design.md insists on is two channels.** |
 | head plate, kept frames | node `:head`, `:symbol` per instance, keys on `[:symbol]` at kept frames |
 | `makeXform` face-oval crop | **gone.** Placement is `[:xform :*]` on `:face`; the stage clips |
-| `stabilize` transforms | `[:xform :*]` on `:head` — framed identity, dense, or keyed at kept frames |
+| `stabilize` transforms | dense `[:xform :*]` on `:head`, read through its optional `:anchors` map |
 | registered underlay | not data — a UI layer riding `(world-of resolver :head)` |
 | painted background cel | node per layer, `[:geom :pts]` **framed**, `[:style :color]` framed |
 | `mouth lead` | `:time {:offset k}` on performance nodes only |
 | `exposure` | `:time {:expose n}` on the clip root, inherited |
-| picture fps | `:time {:source-fps s :sample-fps p}` on the clip root, applied after analysis |
+| picture fps | resolver samples marked generated channels at the picture rate; authored keys keep their own time |
 | hand correction | an `:over` layer, `:offset` or `:replace` |
 
 The brow row is the one worth looking at twice. `docs/design.md` argues at length