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.cpp

@@ -38,6 +38,7 @@
 #include <cstdint>
 #include <ctime>
 #include <filesystem>
+#include <fstream>
 #include <string>
 #include <vector>
 
@@ -224,6 +225,21 @@ std::string makeUniqueTag() {
     return std::to_string(static_cast<long long>(now));
 }
 
+// Reads the whole file into a byte buffer. Returns an empty vector on any I/O
+// failure (the caller then leaves contentHash empty — the safe, confirm-eliciting
+// direction for an unreadable file).
+std::vector<std::uint8_t> readFileBytes(const std::string& path) {
+    std::ifstream f(path, std::ios::binary | std::ios::ate);
+    if (!f) return {};
+    const std::streamoff size = f.tellg();
+    if (size <= 0) return {};
+    std::vector<std::uint8_t> bytes(static_cast<std::size_t>(size));
+    f.seekg(0);
+    f.read(reinterpret_cast<char*>(bytes.data()), size);
+    if (!f) return {};
+    return bytes;
+}
+
 }  // namespace
 
 CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
@@ -483,9 +499,17 @@ CaptureResult OfflineRenderBackend::capture(const CaptureRequest& request) {
     s.lengthSeconds = request.endSeconds - request.startSeconds;
     s.captureTempo  = Master_GetTempo();   // BPM at capture time (verified ~4651)
     s.tier          = Tier::Scratch;       // captures land in scratch by default
-    // contentHash left empty for M3: hashing the rendered file is a peaks/M2-
-    // adjacent concern wired in a later milestone. Empty hashes do NOT dedup, so
-    // this is safe (bank_model treats "" as non-participating).
+    // Content hash: FNV-1a over the rendered file bytes so hashReferencedElsewhere
+    // can identify copies in other banks and suppress the last-reference confirm when
+    // another bank still holds the same file. Best-effort: an unreadable file leaves
+    // contentHash empty — the safe, confirm-eliciting direction (bank_model treats
+    // "" as non-participating in dedup, which is the existing fallback semantics).
+    {
+        const std::vector<std::uint8_t> fileBytes = readFileBytes(expectedPath);
+        if (!fileBytes.empty()) {
+            s.contentHash = hashBytes(fileBytes.data(), fileBytes.size());
+        }
+    }
     s.createdTimestamp = static_cast<std::int64_t>(std::time(nullptr));
 
     result.status  = CaptureStatus::Ok;