Q-W5: persist → session/ext_state_io/prune_fs (deletion authority concentrated); one GetProjExtState grow-loop in bridge_marshal (T2-04, ×3 rewired); bank_book JSON codec → bank_book_json via private static nameKey; persist.h stays umbrella. 61/61 green.

CMakeLists.txt

@@ -267,7 +267,9 @@ target_link_libraries(slot_map PRIVATE json)
 #      sample between banks, JSON round-trip + legacy-bank_index→pool migration.
 #      Mirror of bank_model / view_mode_model; wraps BankIndex (bank_model untouched).
 # ---------------------------------------------------------------------------
-add_library(bank_book STATIC src/core/model/bank_book.cpp)
+add_library(bank_book STATIC
+    src/core/model/bank_book.cpp
+    src/core/model/bank_book_json.cpp)
 target_include_directories(bank_book PUBLIC src)
 target_link_libraries(bank_book PUBLIC bank_model)
 target_link_libraries(bank_book PUBLIC slot_map)
@@ -822,8 +824,10 @@ add_test(NAME velocity_curve_tests COMMAND velocity_curve_tests)
 # 2i) Pure VST3-instrument helpers (Phase S1) — NO VST3, NO REAPER, NO SWELL/LICE.
 #     editor_geometry: the IPlugView LICE editor's rectangle layout + hit-test math
 #     (mirror of mode_switch/bank_grid). bridge_marshal: the REAPER VST-host bridge
-#     read marshalling — GetProjExtState result decode + a small JSON string-field
-#     reader (mirror of capture_paths/wav_trim). Both are unit-tested outside the DAW;
+#     read marshalling — GetProjExtState result decode + the ONE grow-loop retry
+#     policy (readProjExtStateGrowing, Q-W5 rider T2-04) shared by the VST bridge
+#     read AND the extension's persist/usage_scan ext-state reads (hence linked into
+#     reaper_reasampler too). Both are unit-tested outside the DAW;
 #     the VST3 shell (shell/instrument/*) that draws/routes/invokes is DAW-verified.
 # ---------------------------------------------------------------------------
 add_library(editor_geometry STATIC src/core/instrument/ui/editor_geometry.cpp)
@@ -1100,7 +1104,9 @@ add_library(reaper_reasampler MODULE
     src/shell/capture/capture_realtime_shell.cpp
     src/shell/capture/capture_realtime_finalize.cpp
     src/core/capture/capture_realtime.cpp
-    src/persist.cpp
+    src/shell/persist/session.cpp
+    src/shell/persist/ext_state_io.cpp
+    src/shell/persist/prune_fs.cpp
     src/shell/panel/panel_audition.cpp
     src/shell/panel/panel_bank_ops.cpp
     src/shell/panel/panel_drag.cpp
@@ -1129,6 +1135,7 @@ add_library(reaper_reasampler MODULE
     src/shell/actions/prune_action.cpp
     src/ingest.cpp
     src/core/model/bank_book.cpp
+    src/core/model/bank_book_json.cpp
     src/core/model/owned_manifest.cpp
     src/shell/actions/drag_out_win.cpp
     src/shell/actions/instrument_drop_win.cpp
@@ -1141,7 +1148,7 @@ add_library(reaper_reasampler MODULE
     src/core/ui/card_drag.cpp
     src/shell/persist/usage_scan.cpp
 )
-target_link_libraries(reaper_reasampler PRIVATE json wire file_bytes bank_model capture_paths peaks bank_grid mode_switch tab_strip view_mode_model insert_plan render_settings batch_capture tail_control capture_realtime bank_book wav_codec owned_manifest prune_reconcile prune_button app_version provenance drag_out instrument_drop theme component_geometry action_bar footer_bar overflow_menu mode_enable tooltip card_meta card_drag assignment_request bank_sync sample_usage)
+target_link_libraries(reaper_reasampler PRIVATE json wire file_bytes bank_model capture_paths peaks bank_grid mode_switch tab_strip view_mode_model insert_plan render_settings batch_capture tail_control capture_realtime bank_book wav_codec owned_manifest prune_reconcile prune_button app_version provenance drag_out instrument_drop theme component_geometry action_bar footer_bar overflow_menu mode_enable tooltip card_meta card_drag assignment_request bank_sync bridge_marshal sample_usage)
 target_include_directories(reaper_reasampler PRIVATE ${SDK_INC} ${WDL_INC})
 # OUTPUT_NAME is channel-derived (Phase V, V4): "reaper_reasampler" (stable, default) or
 # "reaper_reasampler_beta" (beta). REAPER dlopen's any reaper_* module, so both channels'