Add instanced 8625 stage with independent audio controls

docs/animation-model.md

@@ -495,6 +495,39 @@ reading head over the same channel. The resolver keys its caches by the instance
 path, not by node id — which is a detail of `Making it fast` below, and the one
 place symbol nesting is not free.
 
+### Audio placements and controls
+
+Sound is placed on a timeline as a separate `:audio` node. It uses the same
+`:span`, `:time`, and channel representation as a drawn node. A `:linked-to` id
+records which picture instance it was placed with; it does not force the two
+spans or source in-points to match.
+
+```clojure
+{:id :voice-right :kind :audio :parent :root :z "a4"
+ :linked-to :right
+ :source {:footage "f8cace9e-..."}
+ :span [48 260]
+ :time {:mode :map :at 48 :in 0 :rate 1}
+ :channels {[:audio :gain]
+            {:animated? true :interp :linear
+             :keys {48 0.0, 60 1.0, 245 1.0, 259 0.0} :over []}}}
+```
+
+`[:audio :gain]`, `[:audio :pan]`, and `[:audio :rate]` are ordinary scalar
+channels. They may be framed, keyed, or dense; numeric keyed channels can ramp
+linearly. The time map sets the placement's base source rate, and
+`[:audio :rate]` multiplies it. Audio is mixed from the referenced immutable
+footage when the clip opens. The mix is derived output; the saved document holds
+the nodes and channel keys, not another audio file. One audio element plays that
+mix and remains the clock for both sound and picture.
+
+This is also the boundary for a future control surface. A control has a stable
+target, such as a feature's `:verts` setting or an audio node's
+`[:audio :gain]` channel. The UI and a MIDI binding can address both through the
+same control interface. Their update costs differ: gain can be keyed over time;
+changing the number of lip vertices changes topology and must regenerate its
+dense geometry. A topology setting cannot be treated as a per-frame gain curve.
+
 ## Evaluating a frame
 
 ```clojure