Preserve source cadence and sample picture fps after analysis

docs/animation-model.md

@@ -333,11 +333,15 @@ Every node may map the frame it is evaluated at:
 ```clojure
 :time {:mode :inherit}                        ; the default, and almost always right
 :time {:mode :map :expose 2 :offset -1 :rate 1.0 :loop? false}
+:time {:mode :map :source-fps 30 :sample-fps 12} ; root: lower picture cadence
 ```
 
 Three features that look unrelated are this one mechanism:
 
 - **exposure** is `⌊f/n⌋·n`,
+- **picture fps** quantises source time to a chosen picture grid, then reads the
+  latest source pose at or before that time; source analysis and audio keep their
+  original cadence,
 - **mouth lead** is `f + k`,
 - **a symbol instance's timing** is `(f - at)·rate + in`, with optional looping.
 
@@ -463,8 +467,11 @@ world transform of the node it rides:
 
 Composed with image-pixels-to-local — **both axes divided by `imgH`**, never by
 their own dimension — the photo is registered with the shapes by construction,
-and an unregistered underlay is merely decorative. Which frame it shows, the
-current one or the held plate frame, is a UI choice and not a stored one.
+and an unregistered underlay is merely decorative. The tracing editor chooses
+which source frame to show under a cel. That reference choice is independent of
+the finished picture fps and does not change the dense analysis track. A cel can
+therefore use any useful source frame as its drawing reference, even when that
+frame is not one of the displayed picture poses.
 
 A photo that has to sit *between* two drawn layers is the case that would make it
 a `:bitmap` node with an op of its own. Nothing wants that yet: a reference is
@@ -527,6 +534,7 @@ Proof that it covers what exists, not just what is wanted:
 | 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 |
 | hand correction | an `:over` layer, `:offset` or `:replace` |
 
 The brow row is the one worth looking at twice. `docs/design.md` argues at length