feat(persist): M4 bank persistence + Save-As relocation

Serialize BankIndex to project ext state ('reasampler'), reload on project
load, resolve paths project-relative. Save-As copies the bank to the new .rpp;
identity keyed off a minted GUID (not the recycled ReaProject*) so project
switches don't clobber banks. Pure classifyProjectTransition tested.

CMakeLists.txt

@@ -58,6 +58,7 @@ add_test(NAME capture_paths_tests COMMAND capture_paths_tests)
 add_library(reaper_reasampler MODULE
     src/main.cpp
     src/capture.cpp
+    src/persist.cpp
 )
 target_link_libraries(reaper_reasampler PRIVATE bank_model capture_paths)
 target_include_directories(reaper_reasampler PRIVATE ${SDK_INC} ${WDL_INC})