chore(gradle): add wrapper sha256 + parallel/caching flags

Code review on Task 4 flagged:
- distributionSha256Sum missing (F-Droid reproducibility concern)
- gradle.properties lacked org.gradle.parallel/caching (default off)
- inaccurate K2 attribution in a comment

SHA256 fetched live from downloads.gradle.org. Configuration cache
left commented for now until all plugins confirm AGP 9.1 compat.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,7 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 distributionUrl=https\://services.gradle.org/distributions/gradle-9.5.1-bin.zip
+distributionSha256Sum=bafc141b619ad6350fd975fc903156dd5c151998cc8b058e8c1044ab5f7b031f
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME