refactor(decode): address review — trim test sweep, guard --hash

From /pr-review + /simplify:
- Reduce the differential test's random sweep 1M -> 200k. The 1/2-byte fast
  paths are already proven exhaustively and the 3+ byte path is original code,
  so the sweep is supplementary; this keeps the ASan/Debug ctest ~1s.
- Reject --hash without --decode-replay. The fingerprint pass lives inside the
  decode-replay block, so --hash alone silently printed nothing — a footgun for
  a correctness gate. Now errors clearly.

ctest green; decoded-output fingerprint unchanged (0x75b0cc1fbc0457e9).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: facontidavide

cloudini_lib/test/test_field_encoders.cpp

@@ -257,8 +257,11 @@ TEST(FieldEncoders, DecodeVarintMatchesOracleExhaustiveAndRandom) {
 
   // Randomized sweep over the general (3+ byte) path and all truncation
   // lengths, including malformed all-continuation and overflow-edge varints.
+  // The 1- and 2-byte fast paths are already proven exhaustively above and the
+  // 3+ byte path is the original code verbatim, so this sweep is supplementary
+  // coverage of the interior; 200k keeps the ASan/Debug ctest run fast.
   std::mt19937_64 rng(0xC10D1217ULL);
-  for (int iter = 0; iter < 1'000'000; ++iter) {
+  for (int iter = 0; iter < 200'000; ++iter) {
     const size_t len = 1 + (rng() % 12);  // up to 12 bytes (varint64 worst case is 10)
     for (size_t i = 0; i < len; ++i) {
       // Bias toward continuation bytes so we exercise long/overflowing varints.

cloudini_lib/tools/src/mcap_codec_benchmark.cpp

@@ -290,6 +290,12 @@ int main(int argc, char** argv) {
     std::cerr << "Error: --decode-replay requires --mode so the in-memory replay stays bounded\n";
     return 1;
   }
+  if (do_hash && !decode_replay) {
+    // The fingerprint pass lives inside the decode-replay block, so without it
+    // --hash would silently print nothing — a footgun for a correctness gate.
+    std::cerr << "Error: --hash requires --decode-replay (and --mode)\n";
+    return 1;
+  }
 
   if (!std::filesystem::exists(input_file)) {
     std::cerr << "Error: file does not exist: " << input_file << "\n";