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.cpp

@@ -1,9 +1,28 @@
 #include "capture_paths.h"
 
 #include <cassert>
+#include <cstdint>
+#include <cstdio>
 
 namespace reasampler {
 
+std::string hashBytes(const std::uint8_t* data, std::size_t len) {
+    // FNV-1a 64-bit: deterministic, no dependencies, adequate for dedup identity.
+    // Constants from the FNV spec (http://www.isthe.com/chongo/tech/comp/fnv/).
+    constexpr std::uint64_t kOffsetBasis = 14695981039346656037ULL;
+    constexpr std::uint64_t kPrime       = 1099511628211ULL;
+    std::uint64_t h = kOffsetBasis;
+    for (std::size_t i = 0; i < len; ++i) {
+        h ^= static_cast<std::uint64_t>(data[i]);
+        h *= kPrime;
+    }
+    // Format as 16-digit lowercase hex (zero-padded) for a fixed-length string.
+    char buf[17];
+    std::snprintf(buf, sizeof(buf), "%016llx",
+                  static_cast<unsigned long long>(h));
+    return std::string(buf);
+}
+
 std::string normalizeSlashes(const std::string& path) {
     std::string out = path;
     for (char& c : out) {