feat: replace arti-mobile-ex with custom-built Arti native library

The Guardian Project's arti-mobile-ex AAR has three problems:
1. No 16KB page-aligned binaries (required for Google Play)
2. ArtiProxy's stop()+start() causes state file lock conflicts
   (lock is tied to TorClient object lifetime, released only on GC)
3. ~140MB AAR size

Replace with a custom JNI bridge built from Arti source, following
BitChat's proven approach:

Build tooling (tools/arti-build/):
- build-arti.sh: Clones official Arti, compiles with cargo-ndk
  for ARM64 + x86_64, NDK 25+ for 16KB page alignment
- Cargo.toml: Minimal deps with size-optimized release profile
- src/lib.rs: Custom SOCKS5 proxy with proper lifecycle:
  - initialize() creates TorClient once (holds state lock forever)
  - startSocksProxy() binds port and accepts connections
  - stopSocksProxy() aborts listener only (TorClient stays alive)
  This cleanly separates "stop routing traffic" from "destroy client"

Kotlin side:
- ArtiNative.kt: JNI declarations + ArtiLogCallback interface
- TorService.kt: Uses ArtiNative directly, start() initializes +
  starts proxy, stop() only stops proxy (no lock issues)
- TorManager.kt: Restored stop() calls for OFF/EXTERNAL modes
  since our native stop is now safe

Removed: arti-mobile-ex dependency from build.gradle and version catalog

Native libraries must be built separately:
  cd tools/arti-build && ./build-arti.sh

https://claude.ai/code/session_01BApgDd5udqBzMqysSRMpZu

gradle/libs.versions.toml

@@ -25,7 +25,6 @@ fragmentKtx = "1.8.9"
 gms = "4.4.4"
 jacksonModuleKotlin = "2.21.2"
 javaKeyring = "1.0.4"
-artiMobileEx = "1.2.3"
 junit = "4.13.2"
 kchesslib = "1.0.5"
 kotlin = "2.3.20"
@@ -140,7 +139,6 @@ google-mlkit-language-id = { group = "com.google.mlkit", name = "language-id", v
 google-mlkit-translate = { group = "com.google.mlkit", name = "translate", version.ref = "translate" }
 jackson-module-kotlin = { group = "com.fasterxml.jackson.module", name = "jackson-module-kotlin", version.ref = "jacksonModuleKotlin" }
 java-keyring = { group = "com.github.javakeyring", name = "java-keyring", version.ref = "javaKeyring" }
-arti-mobile-ex = { module = "info.guardianproject:arti-mobile-ex", version.ref = "artiMobileEx" }
 junit = { group = "junit", name = "junit", version.ref = "junit" }
 kchesslib = { module = "io.github.cvb941:kchesslib", version.ref = "kchesslib" }
 kotlinx-collections-immutable = { group = "org.jetbrains.kotlinx", name = "kotlinx-collections-immutable", version.ref = "kotlinxCollectionsImmutable" }