feat(capture): bindable capture family — master/tracks/items/razor (M7)

Four wet capture actions route to OfflineRenderBackend (snapshot/restore
RENDER_*, 32-bit float), produce a Sample, add to the bank, persist. Pure
render_settings maps source mode to RENDER_SETTINGS and parses P_RAZOREDITS
(tested). No arrange insertion. True dry deferred to M10.

src/capture.h

@@ -14,6 +14,7 @@
 // without dragging the SDK into every include site.
 
 #include <string>
+#include <vector>
 
 #include "bank_model.h"
 
@@ -40,10 +41,20 @@ struct CaptureRequest {
     double startSeconds = 0.0;
     double endSeconds   = 0.0;
 
-    // 1.0 = fully wet, 0.0 = fully dry. M3 renders the wet master mix (1.0);
-    // dry/partial routing is M7. Carried now so the Sample records it.
+    // 1.0 = fully wet, 0.0 = fully dry. All M7 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
+    // designed alongside the M10 null test. Also recorded on the Sample.
     double wetDry = 1.0;
 
+    // Track GUID(s) the capture came from, when the source mode is track-scoped
+    // (SelectedTracks). Empty for master/items/razor. The action layer (M7)
+    // resolves the selection to canonical GUID strings and passes them here; the
+    // backend copies them onto the Sample (it does NOT itself read the selection —
+    // it stays source-agnostic, driven entirely by the request).
+    std::vector<std::string> trackGuids;
+
     // Render tail. Default OFF for the spike (exact bounds, no added silence —
     // precision invariant). M7 makes this bindable.
     bool   renderTail   = false;
@@ -90,9 +101,12 @@ public:
     virtual CaptureResult capture(const CaptureRequest& request) = 0;
 };
 
-// Deterministic offline-render backend. M3 implements ONLY the
-// TimeSelection / MasterMix case (both map to "render the master mix over the
-// requested bounds"); any other source mode returns UnsupportedMode.
+// 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).
 class OfflineRenderBackend : public ICaptureBackend {
 public:
     CaptureResult capture(const CaptureRequest& request) override;