Fix Gradle build directory cache for UP-TO-DATE incremental builds.

Cache project .gradle/ and root build/ (execution history + outputs), restore git mtimes after checkout, skip set-version when unchanged, and bump cache keys to v2 with build-jar restore fallback.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/actions/save-gradle-build-cache/action.yml

@@ -1,5 +1,5 @@
 name: Save Gradle build directories
-description: Save module build/ trees after a Gradle build
+description: Save project .gradle/, build/ trees after a Gradle build
 
 inputs:
   primary-key:
@@ -14,6 +14,8 @@ runs:
       with:
         key: ${{ inputs.primary-key }}
         path: |
+          .gradle
+          build
           app/desktop/build
           app/shared/build
           app/android/build