io-core: add androidNativeArm32/Arm64 targets (posix nativeMain already covers them)

[michalharakal]

Summary

skainet-io-core builds every native target (iosArm64, iosSimulatorArm64, macosArm64, linuxX64,
linuxArm64) but not androidNativeArm32 / androidNativeArm64, which blocks on-device
(Android/edge NPU — 32-bit ARM SoCs, arm64 phones) consumers that need engine tokenizers
(TokenizerFactory, SentencePieceTokenizer, byte-level BPE decode).

Why it's trivial

• The native RandomAccessSource actual is PosixPreadRandomAccessSource (posix pread) in a
  shared nativeMain — androidNative supports posix pread.
• commonMain deps are only skainet-lang-core (already androidNative) + kotlinx
  (io-core / coroutines / serialization-json — all androidNative-capable).

So the targets just need declaring; no new actuals.

Verification

./gradlew :skainet-io:skainet-io-core:compileKotlinAndroidNativeArm64 and …Arm32 compile clean.

Not included

skainet-io-safetensors also lacks androidNative but additionally depends on
skainet-compile-core / skainet-compile-dag (not androidNative), so it needs a separate,
larger effort. On-device weight loading for compiled models bakes weights into the vmfb, so
io-core (tokenizer decode) is the piece actually needed on device.

[michalharakal]

PR #836 adds androidNativeArm64 (clean — 64-bit joins the existing posix nativeMain).

androidNativeArm32 needs a follow-up: posix ssize_t/size_t are Int on 32-bit vs Long on the 64-bit targets, so PosixPreadRandomAccessSource can't share nativeMain metadata across both widths (compileNativeMainKotlinMetadata fails with "numbers with different bit widths"). Fix: move the posix source into a 64-bit-only intermediate source set (e.g. native64Main) so the arm32 leaf compiles without it (nothing in commonMain references it — only its own nativeTest and external file-I/O consumers do). Leaving #835 open to track the arm32 half.