fix: avoid zstd segfault in vendored mcap writer

Replace the vendored MCAP Zstd chunk writer's ZSTD_compress2-based path with ZSTD_compressCCtx and keep the selected compression level on the wrapper state.

The previous implementation crashed in Debug and real zed_svo_to_mcap runs on this machine when the first compressed chunk was flushed with --mcap-compression zstd. The same exports succeeded with none and lz4, which narrowed the failure to the shared vendored MCAP Zstd path rather than exporter sync logic.

Also extend mcap_multi_record_tester to accept a compression argument so the Zstd path can be exercised directly during regression testing.

Verified with:
- ./build-debug/bin/mcap_multi_record_tester /tmp/mcap_multi_zstd_test.mcap zstd
- ./build-debug/bin/zed_svo_to_mcap --input jump/experiment/2/2026-03-18T11-27-15/2026-03-18T11-27-15_zed1.svo2 --output /tmp/zed1_single_zstd_fixed.mcap --mcap-compression zstd --end-frame 1
- ./build-debug/bin/zed_svo_to_mcap --segment-dir jump/experiment/2/2026-03-18T11-27-15 --output /tmp/multi_zstd_fixed.mcap --mcap-compression zstd --end-frame 1

src/testers/mcap_multi_record_tester.cpp

@@ -13,7 +13,9 @@
 #include <cstdint>
 #include <filesystem>
 #include <map>
+#include <optional>
 #include <string>
+#include <string_view>
 #include <vector>
 
 namespace {
@@ -31,6 +33,20 @@ constexpr int exit_code(const TesterExitCode code) {
 	return static_cast<int>(code);
 }
 
+[[nodiscard]]
+std::optional<cvmmap_streamer::McapCompression> parse_compression(std::string_view raw) {
+	if (raw == "none") {
+		return cvmmap_streamer::McapCompression::None;
+	}
+	if (raw == "lz4") {
+		return cvmmap_streamer::McapCompression::Lz4;
+	}
+	if (raw == "zstd") {
+		return cvmmap_streamer::McapCompression::Zstd;
+	}
+	return std::nullopt;
+}
+
 }
 
 int main(int argc, char **argv) {
@@ -43,13 +59,21 @@ int main(int argc, char **argv) {
 		argc > 1
 			? std::filesystem::path(argv[1])
 			: std::filesystem::temp_directory_path() / "cvmmap_streamer_multi_record_test.mcap";
+	const auto compression =
+		argc > 2
+			? parse_compression(argv[2]).value_or(cvmmap_streamer::McapCompression::None)
+			: cvmmap_streamer::McapCompression::None;
+	if (argc > 2 && !parse_compression(argv[2])) {
+		spdlog::error("invalid compression '{}': expected none|lz4|zstd", argv[2]);
+		return exit_code(TesterExitCode::CreateError);
+	}
 	if (output_path.has_parent_path()) {
 		std::filesystem::create_directories(output_path.parent_path());
 	}
 
 	auto sink = cvmmap_streamer::record::MultiMcapRecordSink::create(
 		output_path.string(),
-		cvmmap_streamer::McapCompression::None);
+		compression);
 	if (!sink) {
 		spdlog::error("failed to create MCAP sink: {}", sink.error());
 		return exit_code(TesterExitCode::CreateError);