fix(native): restore the windows-only taskkill command path

[leynier]

Change

The v0.96.0 release failed twice on build app windows (runs 36015803627 and 36037904294) with MSB8066 ... alera_native_cargokit ... exited with code -1. Flutter hides cargo's output without -v; a verbose local build shows the real error:

error[E0425]: cannot find function `windowless_async_command` in this scope
  --> src\api\process_session.rs:273:27

#858 removed use alera_core::child_process::windowless_async_command; from rust/src/api/process_session.rs because it looked unused on Linux, but its only caller is the #[cfg(windows)] process tree kill in terminate_invocation. This calls it by its full path so the import cannot go stale on non-Windows hosts again.

Validation

• flutter build windows --release --dart-define=ALERA_FLAVOR=release -v on a Windows 11 machine (Flutter 3.47.2, Rust 1.98.0, Vulkan SDK 1.4.350.0, CI-style R:\c cargokit scratch dir): failed with the error above before the fix, exits 0 after it.
• cargo clippy -p alera_native --all-targets -- -D warnings on Linux passes.

Risk

None beyond the one-line path change. PR CI never compiles alera_native for Windows, so this class of cfg(windows) break only surfaces at release time; a Windows cargo check -p alera_native in pr.yml would catch it earlier.

[pullfrog]

✅ No new issues found.

Reviewed changes

Restored the Windows-only taskkill spawn in terminate_invocation so alera_native compiles after #858 dropped the Linux-unused import.

• Fully qualified windowless_async_command — the #[cfg(windows)] process-tree kill now calls alera_core::child_process::windowless_async_command("taskkill.exe"), matching other Windows-only call sites and keeping the name out of a file-level use that clippy on Linux would strip again.

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