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

src/core/package/CLAUDE.md

@@ -93,6 +93,10 @@ both have landed.
   file size in; header/manifest/layout out), and `requiredPrefixSize` (the
   incremental-read seam for the shell). Framing rides `core/wire/bytes.h`.
 
+`package_compat_tests` is declared here with no library of its own: it decodes the
+frozen `.rsbank` corpus at `tests/fixtures/package_compat/`, whose README owns the
+append-only rule and the per-fixture inventory.
+
 ## Gotchas
 
 - Enums nested inside the `BankModel` blob follow `bank_model`'s own rule — an

src/core/package/CMakeLists.txt

@@ -18,6 +18,12 @@ reasampler_pure_library(bank_package
 # app_version: the tests pin the stamped writer semver against stampVersion().
 reasampler_test(bank_package LINK bank_package app_version)
 
+# The frozen compatibility corpus, decoded rather than regenerated. Fixture path: see
+# tests/package_fixtures.h.
+reasampler_test(package_compat LINK bank_package)
+target_compile_definitions(package_compat_tests PRIVATE
+    REASAMPLER_PACKAGE_FIXTURE_DIR="${REASAMPLER_TESTS_DIR}/fixtures/package_compat")
+
 reasampler_pure_library(import_plan
     SOURCES import_plan.cpp
     LINK PUBLIC package_manifest bank_book origin_ledger PRIVATE package_format capture_paths)