Publish mingwX64 artifacts — enable Windows-native Kotlin/Native consumers #911

Problem

SKaiNET's KMP modules publish linuxX64, linuxArm64, macosArm64, iOS and androidNative targets — but no mingwX64 (and no macosX64). A Kotlin/Native application targeting Windows cannot resolve skainet-lang-core, skainet-backend-cpu, or any skainet-io-* artifact, so a Windows-native binary using SKaiNET is impossible today; the only Windows story is the JVM.

Root cause / scope

Target lists are declared per-module in each build.gradle.kts (#902 — default hierarchy template — and #804 — a multiplatform convention plugin — would centralize this). The pure-Kotlin modules (lang-*, backend-cpu, compile-*, io-*, data-*) show no POSIX-specific code in their target matrix; expected work:

• add mingwX64() to the shared target list,
• provide expect/actual fills where linux/macos actuals exist (e.g. PlatformCpuOpsFactory.*, file access in io modules — kotlinx-io supports mingwX64),
• a Windows CI lane (cross-compilation from Linux hosts works for mingwX64; tests need a Windows runner or Wine).

Proposal

1. Add mingwX64 to the convention target list for skainet-lang-*, skainet-backend-api, skainet-backend-cpu, skainet-io-core/-gguf/-safetensors, skainet-data-api/-simple, and skainet-bom.
2. Scalar CPU kernels first (parity with other K/N targets); SIMD is a separate concern (Vectorized eager CPU kernels for Kotlin/Native targets (native RTF 2.7–5.8x JVM on identical workloads) #910).
3. CI: compile-only gate on Linux cross-compile + a nightly Windows test lane.

macosX64 is deliberately out of scope (Apple-Silicon-only decision downstream); it would ride the same convention change if ever wanted.

Why now

whisper-cli (Whisper + Moonshine ASR CLI on eager CPU; the downstream consumer behind #910) ships native executables for linuxX64/linuxArm64 with byte-identical-to-JVM transcripts, and needs Windows as a first-class native target — a concrete, testable consumer for the new artifacts.

Acceptance

• sk.ainet.core:skainet-lang-core, :skainet-backend-cpu, :skainet-io-gguf resolve for mingwX64 from Maven Central (or snapshots).
• A hello-tensor K/N mingwX64 executable (create tensor, matmul, read back) links and runs on Windows x64.
• Downstream: whisper-cli links its mingwX64 CLI binary and produces a correct golden transcript on Windows.