[Java] `darwin-x64`, `linuxmusl-x64`, or `linuxmusl-arm64`: Embed Rust-based Copilot CLI Runtime and cease requiring Node.js

Predecessor #2308

@roji wrote:

> Java does not currently publish macOS x64 or Linux musl classifiers - but we'll add those soon enough (so better to add these now while we're at it)

Work items in this **Epic** address this gap.

**Confirmed — the assertion is true for the current checkout** (`d5c9d06`, September 7, 2026).

Java publishes these native classifiers:

- `linux-x64` and `linux-arm64` — **glibc only**
- `win32-x64` and `win32-arm64`
- `darwin-arm64`

There is no `darwin-x64`, `linuxmusl-x64`, or `linuxmusl-arm64` build/profile in `java/copilot-native/pom.xml` or either Java publishing workflow. The release summary explicitly lists only those five classifiers in `.github/workflows/java-publish-maven.yml:625-633`.

Additional confirmation:

- `java/copilot-native/scripts/validate-native-host.mjs:6-20` rejects musl Linux hosts.
- `java/README.md:604` explicitly identifies Intel macOS and Linux musl as unsupported.
- Although `validate-native-artifact.mjs` accepts macOS x64 and musl-shaped classifier names for generic artifact validation, no Maven profile or publication job produces them.