Phase 2: Android NDK build

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Part of Phase 2 — Reach (ROADMAP.md).

Validate building OscTap with the Android NDK toolchain (the original C++17-floor rationale explicitly cites "embedded/older-NDK cross-compilers").

• NDK CMake toolchain build of the core + tests.
• Confirm the ip/ POSIX backend builds (or document lwIP/platform-socket guidance, as for the Pico).

Demand-gated. Pursue when an actual Android consumer appears.

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Groundwork landed in #20 (branch claude/phase-2-prep-h8q9mz).

• android/osctap_jni.cpp — JNI bridge exposing the header-only core to Kotlin (buildMessage(address, args[]) → byte[], describe(byte[]) → String). Compile-verified against a real jni.h + the OscTap core.
• android/CMakeLists.txt — NDK build for libosctap_jni.so (per-ABI; -fexceptions so untrusted-input parsing can be caught, not fatal).
• android/OscTap.kt — Kotlin facade + a java.net.DatagramSocket UDP transport.
• The integration tutorial (docs/INTEGRATION_PI5_PICO_ANDROID.md) covers the NDK path and a simpler pure-JVM OSC-library alternative for app devs.

Still open: a full Gradle sample app, and NDK CI (needs the NDK image — weigh against the ROADMAP's standing-surface caution). The NDK build itself was not run in this sandbox (no NDK installed), so the CMake/Gradle recipe is unverified end-to-end.

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