Make Rust build F-Droid compatible

[kasnder]

Summary

• pin Rust 1.95.0 and cargo-ndk 4.1.2
• provision Android Rust targets and fetch locked crates before the offline build phase
• make the Gradle WireGuard bridge build locked and offline
• use the same setup in CI
• document the fdroiddata release workflow in FDROID-UPDATE.md

Why

F-Droid's current TrackerControl recipe only provisions NDK r27c. The new Rust WireGuard bridge additionally needs a pinned Rust toolchain, Android targets, cargo-ndk, and a populated Cargo cache before F-Droid disables networking for the actual build.

Validation

• 21 Rust unit tests passed with --locked --offline
• all four Android Rust ABIs compiled via Gradle with --offline
• ./gradlew :app:assembleFdroidRelease --offline succeeded
• verified the APK contains libwgbridge.so for armeabi-v7a, arm64-v8a, x86, and x86_64
• verified 16 KiB ELF LOAD alignment for all four libraries