Symbols, not timelines; no symbol is special

Everything that holds nodes is a symbol (domain/timeline -> domain/symbol,
:timelines -> :symbols) and a node that places one is :kind :instance. The
reserved :main root is gone: which symbol is on screen is editor state
([:ui :open]), every domain function that needs a symbol is told which, and
a document opens on the longest symbol nothing else places.

Saved projects move to schema 2 through migration 0007, which rewrites leaf
paths, instance kinds and the feature :symbol key; the client refuses a
schema it does not read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/animation-model.md

@@ -415,9 +415,11 @@ different rules:
   *not* to the plate, which is the whole point of it — so the offset genuinely
   belongs at the node, not the clip.
 
-## Timelines, and why a scene is one
+## Symbols, and why a scene is one
 
-A **timeline** is an ordered bag of nodes in its own frame space:
+A **symbol** is an ordered bag of nodes in its own frame space. (Earlier drafts
+and code called this a *timeline*; that word now means only the UI pane that
+shows one.)
 
 ```clojure
 {:frames  91
@@ -427,9 +429,10 @@ A **timeline** is an ordered bag of nodes in its own frame space:
 
 That is the whole type, and **everything that holds nodes is one of these**:
 
-- a clip's **scene** is its root timeline,
-- a **symbol** in the library is a timeline,
-- a node with `:kind :symbol` is an **instance** of one.
+- what a document opens on is a symbol, and **no symbol is reserved** — a new
+  document's is called `main` only because it has to be called something,
+- anything placed inside another symbol is a symbol,
+- a node with `:kind :instance` is an **instance** of one.
 
 An earlier draft of this document had a scene and a `:kind :timeline` symbol as
 two structures with the same fields and never said they were the same thing.
@@ -474,7 +477,7 @@ for all three is the same — **their own**:
 
 ### Instances
 
-A node with `:kind :symbol` and `:of :sym/blink` places one. Its own channels
+A node with `:kind :instance` and `:of :sym/blink` places one. Its own channels
 compose *over* the symbol's, so one definition is placed many times and tinted,
 offset or retimed at each placement — that is how a three-frame blink is reused
 at frames 40, 88 and 200 without copying it.

docs/architecture.md

@@ -639,10 +639,10 @@ is what step 9 implemented, for the subset that exists:
 clip/<cid>/name                     clip/<cid>/subject/<sid>
 clip/<cid>/timing                   clip/<cid>/feature/<fid>
 clip/<cid>/stage                    clip/<cid>/group/<gid>
-clip/<cid>/source                   clip/<cid>/timeline/<tid>
-clip/<cid>/timeline/<tid>/node/<nid>
-clip/<cid>/timeline/<tid>/measured/<nid>
-clip/<cid>/timeline/<tid>/channel/<nid>/<prop>
+clip/<cid>/source                   clip/<cid>/symbol/<sid>
+clip/<cid>/symbol/<sid>/node/<nid>
+clip/<cid>/symbol/<sid>/measured/<nid>
+clip/<cid>/symbol/<sid>/channel/<nid>/<prop>
 ```
 
 Settings live on subject, feature and group leaves. Each feature has one area, so
@@ -785,12 +785,12 @@ clip/:cid/timing                    clip rate
 clip/:cid/subject/:sid              tracked subject and settings
 clip/:cid/feature/:fid              tracked feature and settings
 clip/:cid/group/:gid                shared settings for an eye pair
-clip/:cid/timeline/:tid             frame count, palette
-clip/:cid/timeline/:tid/node/:nid   one node: parent, stencil, z, time
-clip/:cid/timeline/:tid/channel/:nid/:prop
-clip/:cid/timeline/:tid/measured/:nid
-clip/:cid/timeline/:tid/cel/:nid/:frame
-clip/:cid/timeline/:tid/overrides/:nid/:prop
+clip/:cid/symbol/:sid             frame count, palette
+clip/:cid/symbol/:sid/node/:nid   one node: parent, stencil, z, time
+clip/:cid/symbol/:sid/channel/:nid/:prop
+clip/:cid/symbol/:sid/measured/:nid
+clip/:cid/symbol/:sid/cel/:nid/:frame
+clip/:cid/symbol/:sid/overrides/:nid/:prop
 ```
 
 Each feature and node has its own leaf, so tuning separate features and adding

docs/multi-face-representation.md

@@ -8,11 +8,11 @@ symbol instance. Timelines already provide local node names, independent playbac
 and persistence. No new kind of scene container is needed.
 
 ```clojure
-:timelines
+:symbols
 {:main   {:nodes {:root   {:time {:mode :map :expose 2}}
                   :face   {:parent :root :channels <source-to-stage placement>}
-                  :face-1 {:kind :symbol :of :face-1 :parent :face :z "a0"}
-                  :face-2 {:kind :symbol :of :face-2 :parent :face :z "a1"}}}
+                  :face-1 {:kind :instance :of :face-1 :parent :face :z "a0"}
+                  :face-2 {:kind :instance :of :face-2 :parent :face :z "a1"}}}
  :face-1 {:nodes {:head {...} :mouth {:parent :head ...} ...}}
  :face-2 {:nodes {:head {...} :mouth {:parent :head ...} ...}}}
 

docs/timing-handoff.md

@@ -70,7 +70,7 @@ handling and the relevant key whitelist if its storage location requires it.
 - `freeze/performance-nodes` marks generated animated channels with
   `:pose-sampled?` and local `:pose-group` names. This includes keyed visibility
   as well as dense geometry. `:generated` remains provenance for regeneration.
-- `timeline/channel-frame` already applies explicit pose choices and default
+- `symbol/channel-frame` already applies explicit pose choices and default
   picture sampling to marked channels. Playback and export both use
   `clip/resolver` with `:picture-fps`; there is no need for a second sampling
   implementation. Export's pose count is still a rate-based estimate.