Preserve source cadence and sample picture fps after analysis

docs/architecture.md

@@ -49,6 +49,11 @@ overrides and kept-frame sets are all in clip-frame space, so a clip slides on
 the timeline without a single stored number changing. Exposure and lead are
 transforms *within* clip space:
 
+Detection retains every source frame. A chosen picture fps samples the frozen
+roto in clip time; it changes neither source-frame count nor the audio clock.
+The set of source frames an artist uses as cel tracing references is another
+selection, independent of the picture fps.
+
 ```clojure
 (defn pose-frame [clip cf]
   (-> cf (expose (:exposure clip)) (shift (:lead clip) (count-frames clip))))
@@ -73,7 +78,7 @@ project
 │   ├── scene                node tree
 │   ├── channels             node+property -> keyframe stream, in cf
 │   ├── overrides            (node, property, cf) -> value, applied last
-│   └── cels                 painted vector layers, per exposure slot
+│   └── cels                 painted vector layers, keyed by chosen clip frames
 └── sequence[]               clip placements: {clip-id, at, in, out}
 ```
 
@@ -81,6 +86,10 @@ project
 it a name is most of the work: `state` in `app.js` is a clip with its analysis
 inlined and its palette global.
 
+Cel keys select where drawings begin and how long they hold. The source frames
+shown beneath a cel while tracing are chosen independently, and picture fps
+only controls which analyzed pose the finished roto displays at a given time.
+
 ### Two things called "track"
 
 `docs/design.md` says "dense track" for the landmark stream. A timeline also