fix: address review findings on prewarm gating and gitignore

Revalidate the section under RenderLock before the idle prewarm scan,
add a largest-block floor to its heap gate, rebuild the differential
baseline when the grayscale scratch allocation fails after an async
refresh, and anchor generated-file gitignore patterns to the repo root.

.gitignore

@@ -27,7 +27,7 @@ lib/EpdFont/scripts/output/
 !.claude/skills/
 /managed_components
 /.dummy
-CMakeLists.txt
-dependencies.lock
-sdkconfig.default
-sdkconfig.defaults
+/CMakeLists.txt
+/dependencies.lock
+/sdkconfig.default
+/sdkconfig.defaults