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.properties

@@ -10,8 +10,14 @@ kotlin.code.style=official
 # Enables namespacing of each library's R class, less RAM, faster builds.
 android.nonTransitiveRClass=true
 
-# Use new K2 compiler defaults (Kotlin 2.x).
+# Kotlin incremental compilation (default; kept explicit for documentation).
 kotlin.incremental=true
 
 # Reproducible builds for F-Droid.
 android.uniquePackageNames=true
+
+# Performance: enable parallel project execution and build cache (stable in Gradle 9).
+org.gradle.parallel=true
+org.gradle.caching=true
+# Configuration cache: compatible with AGP 9.1; opt in when all plugins confirm support.
+# org.gradle.configuration-cache=true