feat(codegen): native Kotlin/JNI generator + Android example over the C ABI

[Ivansete-status]

Adds an Android example consuming the timer library's native (zero-
serialization) C ABI from Kotlin via JNI. Based on the native-FFI stack (#58) so
it can use typed struct returns.

What

• examples/timer/android/ — an Android library module with an idiomatic Kotlin
  TimerNode class.
• jni/my_timer_jni.c — a JNI shim that turns each Kotlin external fun into a
  blocking call into the native ABI and reads the typed EchoResponse struct out
  of the result callback.
• build-libs.sh cross-compiles, per ABI (arm64-v8a + x86_64), both
  libmy_timer.so (the Nim library) and libmy_timer_jni.so (the bridge) into
  src/main/jniLibs/, which Gradle packages automatically. An instrumented test
  covers create / version / echo.

Validation

Native build verified for both ABIs: correct aarch64/x86_64 ELF, the 4
Java_org_logos_mytimer_* symbols exported, and libmy_timer_jni.so correctly
NEEDS libmy_timer.so. (Kotlin/Gradle layer runs on a device/emulator.)

Separate PR, as requested. Sibling to the iOS example PR; both target the stack
tip and retarget to master once it lands.

🤖 Generated with Claude Code

[Ivansete-status]

Expanded scope: this PR now also adds the Kotlin/JNI generator that produces the wrapper, not just the hand-written example (mirrors what #59 did for Swift).

• ffi/codegen/kotlin.nim — from one set of {.ffi.} metadata it emits both artifacts: the idiomatic Kotlin AutoCloseable wrapper (MyTimerNode.kt) and the JNI shim (my_timer_jni.c). Wired into the targetLang dispatch + a genbindings_kotlin nimble task.
• The hand-written TimerNode.kt / my_timer_jni.c are replaced by generated output; the instrumented test points at the derived MyTimerNode.
• Callback-shape selection (ack / string / struct) is per proc; the shim blocks on a condvar until the FFI-thread callback fires. An all-string struct return crosses as a String[] repacked into a data class.
• Procs needing seq/Option params, multiple struct params, or a non-string struct return (complex, schedule) are skipped with a logged notice so the generated code always compiles — that marshaling, events, and async are the next increments.

Validated by cross-compiling both ABIs with the NDK: arm64-v8a + x86_64 build clean, the four Java_org_logos_mytimer_MyTimerNode_* symbols are exported, and libmy_timer_jni.so correctly NEEDS libmy_timer.so.

🤖 Generated with Claude Code