Make a bake's Banked landing reachable only through the persist upgrade

The dedup path claimed it directly, so a key could be answered Ok pointing at an entry a failed persist never wrote. Write-back is now guarded and three-valued, and landing one request split out of the scan pass.

src/app/CMakeLists.txt

@@ -11,6 +11,7 @@ add_library(reaper_reasampler MODULE
     ${REASAMPLER_SRC_DIR}/shell/capture/capture_orchestrator.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/capture_batch.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/bake_land.cpp
+    ${REASAMPLER_SRC_DIR}/shell/capture/bake_landing.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/scope_resolve.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/render_selection.cpp
     ${REASAMPLER_SRC_DIR}/shell/capture/render_isolation.cpp