feat(bank_model): add Phase S seam fields (rootNote + loop points) to Sample

Additive optional MIDI root note and sustain-loop points with JSON round-trip and deserialize-boundary validation; capture leaves them empty (not derivable). Mirrors the provenance addition; BankIndex behavior unchanged.

src/capture.cpp

@@ -530,6 +530,11 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
         }
     }
     s.createdTimestamp = static_cast<std::int64_t>(std::time(nullptr));
+    // Phase S seam fields (rootNote / loop) left empty (D-B). An offline render of a
+    // master mix / track / time-selection is not a single played note, so no root
+    // note is derivable here — we do NOT guess one. Loop points are set later by an
+    // explicit user action, not at capture. Leaving them empty is the honest default;
+    // the instrument (Phase S) treats an absent root note as "not a pitched sample".
 
     result.status  = CaptureStatus::Ok;
     result.sample  = s;