fix(capture): populate contentHash on captured samples to fix always-prompting remove

contentHash was left empty on every capture; hashReferencedElsewhere returns
false for empty hashes, so every remove looked like a last reference. Add
FNV-1a hashBytes to capture_paths, wire into both commit paths. NOTE: this
activates index dedup-by-hash in production — a bit-identical re-capture now
collapses onto the existing entry instead of adding a duplicate (spec-intended).

src/realtime_record.cpp

@@ -52,7 +52,10 @@ Sample sampleFromRecordedCapture(const RecordedCapture& cap) {
     s.lengthSeconds = cap.endSeconds - cap.startSeconds;
     s.captureTempo  = cap.captureTempo;
     s.tier          = Tier::Scratch;    // captures land in scratch by default
-    // contentHash left empty: empty hashes do not participate in dedup (bank_model).
+    // contentHash set by the caller (capture_realtime.cpp) after the file is
+    // 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.
     s.createdTimestamp = cap.createdTimestamp;
     return s;
 }