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/capture_paths.h

@@ -12,6 +12,8 @@
 // the filesystem. The bank subfolder name is a fixed constant so the same
 // project always resolves the same bank location (determinism).
 
+#include <cstddef>
+#include <cstdint>
 #include <string>
 
 namespace reasampler {
@@ -31,6 +33,15 @@ struct BankPaths {
     std::string fileStem;       // <stem>                        (RENDER_PATTERN — REAPER appends the extension)
 };
 
+// Computes a deterministic FNV-1a 64-bit content hash over `len` bytes at `data`
+// and returns it as a 16-character lowercase hex string. Designed to fill
+// Sample::contentHash so the confirm-on-last-reference guardrail
+// (BankBook::hashReferencedElsewhere) can distinguish "no other bank holds this
+// file" from "another bank holds the same file." An empty buffer returns the bare
+// FNV-1a 64-bit offset basis in hex (a stable, non-empty sentinel that two empty
+// files would share, but real WAV files are never empty).
+std::string hashBytes(const std::uint8_t* data, std::size_t len);
+
 // Normalizes a path to forward slashes and strips any trailing slash. Empty in
 // -> empty out. Pure string transform (does not consult the filesystem).
 std::string normalizeSlashes(const std::string& path);