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.h

@@ -63,6 +63,22 @@ struct Levels {
     bool operator==(const Levels& o) const;
 };
 
+// Sample-accurate sustain-loop bounds, as frame indices into the captured file
+// (Phase S seam field, D-B). A bank intrinsic — a fact about the file, like
+// sampleRate or length — consumed by the future MIDI-playback instrument to hold
+// notes past the recorded length. Modeled as one optional struct (not two loose
+// optionals) so "both points or neither" is a structural invariant, not a rule to
+// re-check at every boundary. Frame indices, not seconds, because the loop is a
+// per-sample-frame contract; the instrument reads the file's sample rate to relate
+// them to time. Invariant (enforced at the deserialize boundary): 0 <= start <= end.
+// start == end is a valid zero-length loop marker.
+struct LoopPoints {
+    std::int64_t start = 0;
+    std::int64_t end   = 0;
+
+    bool operator==(const LoopPoints& o) const;
+};
+
 // The metadata record for one captured sample. The audio itself lives in a
 // project-relative file; `relativePath` is ALWAYS relative (enforced at the
 // BankIndex::add boundary — see AddResult).
@@ -95,6 +111,18 @@ struct Sample {
 
     std::optional<std::string> key;  // musical key, when known
 
+    // Phase S seam fields (D-B) — bank intrinsics for the MIDI-playback instrument,
+    // additive like `provenance` (M1). Both default cleanly empty: pre-Phase-S
+    // samples deserialize without them and re-serialize without inventing values.
+    //  - rootNote: MIDI note (0..127) the sample was recorded at, so the instrument
+    //    can repitch it across the keyboard. DISTINCT from the musical `key` above:
+    //    `key` is a human label ("F#m"); `rootNote` is the exact pitch for repitch.
+    //    Populated at/after capture only where derivable — left empty (never guessed)
+    //    when the source is not a single played note.
+    //  - loop: sustain-loop bounds, populated only where explicitly set.
+    std::optional<int>        rootNote;
+    std::optional<LoopPoints> loop;
+
     Levels levels;
     bool   clipped = false;