Auto-save the active render layer's sublayer file

Render layer overrides only reached disk on an explicit File > Save --
SaveDirtyRenderLayers() was called from SaveUsdFile/SaveUsdFileAs and nowhere
else. Everything a user authored after switching to a render layer (property
values, shader parameters, material assignments) sat in a dirty in-memory
SdfLayer marked with an orange asterisk, and a crash took the whole layer's
overrides with it.

Application::Update() now polls the active render layer's IsDirty() and saves
it 0.5s after the user goes idle, gated on IsAnyItemActive() and the left mouse
button so a slider or gizmo drag produces one write on release rather than one
per frame.

Polling rather than hooking CommandHistory::Push/Undo/Redo is deliberate: many
of the writes this is meant to catch never build a command and author straight
to the ambient edit target -- PropertyPanel's live WriteTranslate/Rotate/Scale
and its generic attribute widgets, TransformManipulator's gizmo drags, and
MaterialEditorPanel::RenderInputWidget's shader input writes. A command hook
would have silently missed exactly the property and shader-parameter edits at
issue. IsDirty() catches every path, commanded or not.

Scope is the active layer only. Muted render layers still rely on
SaveDirtyRenderLayers() at save time, and the root layer is never auto-written
-- so the persisted active-layer choice (custom layer data on the root layer)
still only survives a reopen once the stage itself is saved. Auto-writing the
user's main scene file was ruled out.

- SaveRenderLayerIfDirty() refreshes LayerManager after a successful save, or
  the cached isDirty snapshot leaves a stale asterisk in StageEditorPanel and
  the Scene Hierarchy sublayer list. A failed save latches the layer id so a
  read-only file can't spam the log twice a second.
- Switching layers flushes the outgoing one, and Shutdown() flushes before the
  panels are destroyed, so edits inside the debounce window aren't stranded.
- RefreshManagers() clears the auto-save state, whose layer ids belong to the
  outgoing stage.
- New "Auto-save active layer" checkbox in the Render Layer panel, on by
  default, persisted as AutoSaveRenderLayer in preferences.ini.

See docs/adr/0002-render-layer.md D9-D11 for the rationale.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/Application.h

@@ -37,6 +37,8 @@ struct AppPreferences {
     /// customData so they survive reopening the material; when false, that
     /// state is session-only.
     bool keepGraphNodeViewSettingsInUsd = false;
+    /// Auto-save the active render layer's SdfLayer shortly after each edit.
+    bool autoSaveRenderLayer = true;
 };
 
 class Application {
@@ -73,6 +75,20 @@ private:
     /// rest would otherwise be silently dropped on save.
     void SaveDirtyRenderLayers();
 
+    /// Per-frame debounce driving the active render layer's auto-save.
+    /// Polls SdfLayer::IsDirty() rather than hooking CommandHistory because
+    /// many write paths (PropertyPanel's live transform/attribute widgets,
+    /// TransformManipulator's gizmo drags, MaterialEditorPanel's shader input
+    /// widgets) author straight to the ambient edit target without going
+    /// through a command — a command hook would silently miss them.
+    void TickRenderLayerAutoSave(float deltaTime);
+    /// Saves the active render layer's SdfLayer if it's dirty. Returns true
+    /// only when a save actually happened.
+    bool FlushActiveRenderLayerSave();
+    /// Saves one render layer by identifier if it's dirty; no-op for an empty
+    /// id (Default active) or a layer whose previous save failed.
+    bool SaveRenderLayerIfDirty(const std::string& layerId);
+
     // Playblast
     void CapturePlayblastFrame();
 
@@ -120,6 +136,13 @@ private:
     AppPreferences m_prefs;
     bool           m_showPreferences = false;
 
+    // Render-layer auto-save state (see TickRenderLayerAutoSave).
+    float       m_renderLayerAutoSaveTimer = 0.0f;
+    std::string m_renderLayerAutoSaveLastActiveId;
+    // Identifier of a render layer whose Save() failed — retried only after the
+    // active layer changes, so a read-only file can't spam the log every tick.
+    std::string m_renderLayerAutoSaveFailedId;
+
     // OCIO edit buffers for Preferences dialog
     char m_prefOcioDisplayBuf[128]    = {};
     char m_prefOcioViewBuf[128]       = {};