feat(render): node-agent AE snapshot runner (Epic C2) + colour render-binding (Epic B)

C2 — real-AE scene snapshots on the node:
- node-agent: runner/snapshot.go RunSnapshot (aerender -comp <key> -s f -e f
  → findRenderedOutput → ffmpeg -frames:v 1 PNG); client ClaimSnapshot /
  GetSnapshotUploadURL / ReportSnapshotResult / ReportSnapshotFail; snapshotLoop +
  pollSnapshotOnce mirroring the scan loop (reuses the AE-exclusive lock).
- render-svc: GetSnapshotJobMeta + UploadURL handler presigns a PUT to the
  public-read user-uploads bucket at snapshots/{project}/{scene}.png and returns a
  permanent public_url (not the time-limited export presign); MINIO_UPLOAD_BUCKET +
  MINIO_PUBLIC_URL config + compose env + /snapshot/:id/upload-url route.

Epic B — bind edited colours into the render:
- render-svc GetRenderBindings UNIONs studio.saved_shared_colors +
  saved_scene_colors (type 'color') so the node writes them before render.
- node-agent binder.go routes type:"color" bindings into the bind-spec colors[]
  array that bind.jsx already applies to the frshare colour layers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/node-agent/internal/runner/binder.go

@@ -77,9 +77,14 @@ type bindLayerSpec struct {
 	Type  string `json:"type"` // "text" | "media" | "audio"
 	Value string `json:"value"`
 }
+type bindColorSpec struct {
+	Key   string `json:"key"`
+	Value string `json:"value"`
+}
 type bindSpec struct {
 	Comp   string          `json:"comp,omitempty"`
 	Fps    int             `json:"fps,omitempty"`
+	Colors []bindColorSpec `json:"colors,omitempty"`
 	Layers []bindLayerSpec `json:"layers"`
 }
 
@@ -109,6 +114,15 @@ func RunBinder(ctx context.Context, job *Job, workDir string) (string, error) {
 		if key == "" {
 			continue
 		}
+		// Colours bind to frshare/frd_* text layers (hex source text) — bind.jsx
+		// propagates them via template expressions, not as visible layer content.
+		if strings.EqualFold(strings.TrimSpace(b.Type), "color") {
+			if strings.TrimSpace(b.Value) == "" {
+				continue
+			}
+			spec.Colors = append(spec.Colors, bindColorSpec{Key: key, Value: b.Value})
+			continue
+		}
 		if isMediaBinding(b.Type) {
 			if strings.TrimSpace(b.Value) == "" {
 				continue