Phase 27: strengthen undo transactions and improve editor workflow

docs/MIXER.md

@@ -104,7 +104,8 @@ plus a baseline, cloning metadata and graph, never audio samples. Common graph,
 track, clip, note, mixer, import/relink and automation edits are included. One
 accepted gesture is one step; selection, scrolling and graph-node placement are
 GUI state and are not undoable. New/Open resets history; Save retains it.
-Undo/redo stops and resets transport and resets editor selections/layout. The
+Undo/redo stops and resets transport; Phase 27 preserves surviving selections
+by stable ID (layout remains session state). The
 candidate is installed before moving the history cursor: validation, asset-plan
 or backend failure retains the current document and history position. Missing
 assets retain the existing silent/reporting behavior. A new edit after Undo
@@ -112,8 +113,9 @@ removes the redo branch; the oldest state is evicted at capacity. Allocation
 ID high-water marks survive Undo/branching so abandoned IDs are not reused. History is
 session-only and Undo marks the document dirty even if it matches a saved state.
 External WAV files are never deleted or restored by Undo; recording itself cannot
-be undone while active. An allocation failure retaining a new history snapshot
-reports an error; the accepted edit remains in the document.
+be undone while active. Phase 27 reserves snapshot storage before GUI commands mutate/publish, so an
+allocation failure rejects the command before adoption; reserved capture itself
+does not allocate. C clients using unreserved history record must handle errors.
 
 With mixer focus, M/S toggles mute/solo, Left/Right changes gain by 0.05,
 Shift+Left/Right changes pan, and Up/Down selects a track. Native sliders also