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/realtime_record.cpp

@@ -58,6 +58,9 @@ Sample sampleFromRecordedCapture(const RecordedCapture& cap) {
     // finalized and on disk — the hash is over the finished file bytes. Left empty
     // here because sampleFromRecordedCapture runs before the file exists (the
     // mapping is pure / DAW-free); the shell patches it in after the move+trim.
+    // Phase S seam fields (rootNote / loop) left empty (D-B) — same reasoning as the
+    // offline path: a realtime record of wet output is not a single played note, so
+    // no root note is derivable; loop points are set by a later explicit action.
     s.createdTimestamp = cap.createdTimestamp;
     return s;
 }