Rework capture into three FX-scope actions with inferred range

Replace the four capture modes with item/track/master scope actions. Each infers
its range (razor-else-time) and enforces FX scope via non-destructive
FX-bypass-around-render (RAII I_FXEN snapshot/restore over ancestors + master).
Corrects the defect of items captured through parent FX.

src/capture.h

@@ -41,7 +41,7 @@ struct CaptureRequest {
     double startSeconds = 0.0;
     double endSeconds   = 0.0;
 
-    // 1.0 = fully wet, 0.0 = fully dry. All M7 actions set this to 1.0 (wet).
+    // 1.0 = fully wet, 0.0 = fully dry. All three-scope capture actions set this to 1.0 (wet).
     // The field is kept as the seam for future true-dry work (M10 null test):
     // true pre-FX dry offline is NOT available via RENDER_SETTINGS — it requires
     // FX-bypass-around-render or the M8 realtime pre-FX path, and will be
@@ -101,12 +101,12 @@ public:
     virtual CaptureResult capture(const CaptureRequest& request) = 0;
 };
 
-// Deterministic offline-render backend. M7 implements the full offline source
-// family — master mix / time selection, selected tracks, selected items, razor
-// area — all wet-only (render_settings.h) with optional tail. The source
-// selection + range are resolved by the caller (the action layer) and handed in
-// via the CaptureRequest; the backend drives RENDER_* and never reads the DAW
-// selection itself. SourceMode::Realtime returns UnsupportedMode (that is M8).
+// Deterministic offline-render backend. Drives the full offline source family —
+// master mix / time selection, selected tracks, selected items, razor area — all
+// wet-only (render_settings.h) with optional tail. The source selection + range
+// are resolved by the caller (the action layer) and handed in via the
+// CaptureRequest; the backend drives RENDER_* and never reads the DAW selection
+// itself. SourceMode::Realtime returns UnsupportedMode (that is the M8 backend).
 class OfflineRenderBackend : public ICaptureBackend {
 public:
     CaptureResult capture(const CaptureRequest& request) override;