windows: restore stable optimized conversion coverage under LLVM 21

[qartik]

Summary

Windows CI still avoids the normal optimized and cross-target qir_to_qis path because the LLVM 21 Windows lane can hard-crash during optimized codegen / target-machine work.

Current workaround

Windows validation is intentionally reduced to conservative host-target smoke coverage:

• opt_level=0
• host-target conversion (target="native" in the Windows smoke test)
• conversion + bitcode parse smoke coverage instead of exact snapshot matching
• no equivalent optimized aarch64 conversion coverage like Linux and macOS

This is a stability workaround, not the desired steady state.

Evidence

Historical repro from earlier CI rollout:

• CI run: 23202614680
• job: 67428980077
• failure: process abort with 0xc0000005 (Invalid access to memory location)

Recent local Windows Arm64 repros still show that the optimized path is unstable:

• cargo run -- tests/data/base.ll -> 0xc0000005
• cargo run -- -O1 -t native tests/data/base.ll -> 0xc0000005

By contrast, the conservative workaround remains stable:

• cargo test test_snapshot_conversion_windows_smoke -- --nocapture passes
• cargo test -q passes on the Windows O0 path

We also observed earlier that, on Windows, even touching the module triple in the opt_level=0 path could reproduce similar crashes, so src/opt.rs intentionally keeps the Windows O0 path as a no-op after target validation.

Goal

Restore at least one stable Windows test that exercises the real optimized conversion path, ideally matching the non-Windows qir_to_qis(..., opt_level=2, target="aarch64") flow.

Good next steps

• identify the smallest optimized Windows repro that still crashes
• determine whether the fault is target-machine creation, triple/data-layout mutation, pass execution, or teardown
• add a stable Windows optimized regression test if a narrower safe path exists
• remove the platform split in src/convert.rs once Windows can reliably run the optimized path

Done when

• Windows can run at least one stable end-to-end conversion test on the real optimized path
• the host-target O0 workaround is no longer required for CI confidence
• the limitation note can be removed from CI and code comments