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/bank_model.cpp

@@ -35,6 +35,10 @@ bool Levels::operator==(const Levels& o) const {
     return peakDb == o.peakDb && rmsDb == o.rmsDb && lufs == o.lufs;
 }
 
+bool LoopPoints::operator==(const LoopPoints& o) const {
+    return start == o.start && end == o.end;
+}
+
 bool Sample::operator==(const Sample& o) const {
     return id == o.id && displayName == o.displayName && relativePath == o.relativePath &&
            sourceMode == o.sourceMode && sourceRange == o.sourceRange &&
@@ -43,7 +47,8 @@ bool Sample::operator==(const Sample& o) const {
            lengthSeconds == o.lengthSeconds && lengthBeats == o.lengthBeats &&
            captureTempo == o.captureTempo &&
            captureTimeSigNum == o.captureTimeSigNum &&
-           captureTimeSigDenom == o.captureTimeSigDenom && key == o.key && levels == o.levels &&
+           captureTimeSigDenom == o.captureTimeSigDenom && key == o.key &&
+           rootNote == o.rootNote && loop == o.loop && levels == o.levels &&
            clipped == o.clipped && tier == o.tier && contentHash == o.contentHash &&
            provenance == o.provenance && createdTimestamp == o.createdTimestamp;
 }
@@ -253,6 +258,21 @@ void writeSample(std::string& out, const Sample& s) {
     w.keyBegin("key");
     if (s.key) writeEscaped(out, *s.key); else out += "null";
 
+    // Phase S seam fields (D-B). Emitted as null when absent (same shape as `key`
+    // and `provenance`) so pre-Phase-S JSON — which lacks these keys entirely —
+    // parses to empty optionals and re-serializes without invention.
+    w.keyBegin("rootNote");
+    if (s.rootNote) out += numToStr(*s.rootNote); else out += "null";
+
+    w.keyBegin("loop");
+    if (s.loop) {
+        ObjWriter lp(out);
+        lp.keyRaw("start", numToStr(s.loop->start));
+        lp.keyRaw("end", numToStr(s.loop->end));
+    } else {
+        out += "null";
+    }
+
     w.keyBegin("levels");
     {
         ObjWriter l(out);
@@ -608,6 +628,41 @@ bool Parser::parseSample(Sample& s) {
                 if (!parseString(k)) return false;
                 s.key = k;
             }
+        } else if (key == "rootNote") {
+            bool wasNull = false;
+            if (!expectNullOr(wasNull)) return false;
+            if (wasNull) {
+                s.rootNote.reset();
+            } else {
+                int v = 0;
+                if (!parseInt(v)) return false;
+                // Valid MIDI note range: 0..127 inclusive (boundaries valid).
+                if (v < 0 || v > 127) return false;
+                s.rootNote = v;
+            }
+        } else if (key == "loop") {
+            bool wasNull = false;
+            if (!expectNullOr(wasNull)) return false;
+            if (wasNull) {
+                s.loop.reset();
+            } else {
+                if (!consume('{')) return false;
+                LoopPoints lp;
+                do {
+                    std::string lk;
+                    if (!parseKey(lk)) return false;
+                    std::int64_t lv = 0;
+                    if (!parseInt64(lv)) return false;
+                    if (lk == "start") lp.start = lv;
+                    else if (lk == "end") lp.end = lv;
+                } while (consume(','));
+                if (!consume('}')) return false;
+                // Invariant: 0 <= start <= end. start == end is a valid zero-length
+                // marker; a negative index or start > end is malformed, not silently
+                // clamped (mirrors the enum-range rejection above).
+                if (lp.start < 0 || lp.end < lp.start) return false;
+                s.loop = lp;
+            }
         } else if (key == "levels") {
             if (!consume('{')) return false;
             do {