Three stores, three importers and three pickers that differ only in their type are one of each (#85)

Refactoring round one: HttpFontStore/HttpSoundStore/HttpTextureStore collapse into a generic
HttpFileStore over a FileStore contract; fontImport/soundImport/textureImport share assetImport
with an ImportKind per sort; ImagePicker/FontPicker/SoundPicker sit on FilePicker and describe
only their row. The group/comment drag logic leaves GraphCanvas for annotationDrag.ts (with its
own unit test), declaredRows.ts replaces the duplicated row build in particles.ts and shaders.ts,
and dead code in font/sound/compile/emission/transform/actions is gone. README: asset list,
check count 958, the two-job CI, the Scenes wiki page.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/core/commands/require.ts

@@ -1,5 +1,5 @@
 /** Lookups that throw a `CommandError` when the target does not exist. */
-import { findNode, type Graph, type GraphNode } from '../graph/Graph';
+import { findNode, type Graph, type GraphEdge, type GraphNode } from '../graph/Graph';
 import { type NodeDef, nodeDef } from '../graph/NodeRegistry';
 import type { Asset, BehaviorAsset, StateMachineAsset, VariablesAsset } from '../model/Asset';
 import { findObject, type ObjectTree } from '../model/hierarchy';
@@ -98,6 +98,12 @@ export function requireNode(project: Project, assetId: string, nodeId: string):
   return node;
 }
 
+export function requireEdge(project: Project, assetId: string, edgeId: string): GraphEdge {
+  const edge = requireGraph(project, assetId).edges.find((e) => e.id === edgeId);
+  if (!edge) throw new CommandError(`Edge ${edgeId} not found`);
+  return edge;
+}
+
 /** A `CommandError`, so an unknown type becomes a notice instead of a crash. */
 export function requireNodeDef(type: string): NodeDef {
   const def = nodeDef(type);