feat(android): Phase 4 — Rust core to Android .so via JNI

Implement the shared-Rust-core leg of the Android Stage-1 prototype
(docs/superpowers/specs/2026-06-27-android-stage1-prototype-plan.md).

android/rust/catacomb_core is a cdylib that reuses the pure desktop
modules (vtt, error_class, platform) verbatim via #[path] includes — no
logic fork — and exposes them to Kotlin over JNI as String-in/String-out
entry points:

  RustCore.vttParse(vtt)        -> JSON [{start,text},...]
  RustCore.classifyError(log)   -> JSON {class,label,hint}
  RustCore.platformFromUrl(url) -> JSON {dir_name,display_name,icon}
  RustCore.platformDirName(url) -> folder name

Built with panic=abort + per-entry catch_unwind so a Rust panic can never
unwind into the JVM. build.sh locates the NDK portably and cross-compiles
to arm64-v8a + x86_64, depositing the .so into app/src/main/jniLibs/.

Verified: cargo test (38 pass), llvm-readelf confirms all 4 JNI symbols
exported in the arm64 .so, and an end-to-end host-JVM round-trip
(System.load + call each native method) returns correct JSON.

Phases 2/3/5 (WebView-BotGuard anti-bot) remain device-only and untouched.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

android/.gitignore

@@ -0,0 +1,14 @@
+# Rust build artifacts
+rust/catacomb_core/target/
+
+# Compiled native libs land here via rust/catacomb_core/build.sh — rebuild
+# rather than commit binaries.
+app/src/main/jniLibs/
+
+# Gradle / Android build outputs (Phase 1, not yet scaffolded here)
+.gradle/
+build/
+app/build/
+local.properties
+*.iml
+.idea/