Freeze the package compatibility corpus: real .rsbank bytes proving both ladder directions, every truncation site, and the round trip

tests/package_fixtures.h

@@ -0,0 +1,25 @@
+#pragma once
+// The one accessor over the frozen package-compat corpus (tests/fixtures/package_compat).
+// REASAMPLER_PACKAGE_FIXTURE_DIR is a compile-time absolute path defined by each
+// consuming test target: the corpus is source-tree data, and a test's working directory
+// under ctest differs between single- and multi-config generators, so no relative
+// spelling reaches it from both.
+//
+// Every caller must check the returned size: a fixture that failed to open reads as an
+// empty buffer, which a "this must be Malformed" assertion would otherwise pass.
+
+#include <cstdint>
+#include <fstream>
+#include <iterator>
+#include <string>
+#include <vector>
+
+inline std::string packageFixturePath(const std::string& name) {
+    return std::string(REASAMPLER_PACKAGE_FIXTURE_DIR) + "/" + name;
+}
+
+inline std::vector<std::uint8_t> packageFixtureBytes(const std::string& name) {
+    std::ifstream f(packageFixturePath(name), std::ios::binary);
+    return std::vector<std::uint8_t>(std::istreambuf_iterator<char>(f),
+                                     std::istreambuf_iterator<char>());
+}