fix(windows): preserve POSIX metadata under WHPX soak

[ZhiXiao-Lin]

Summary

• add a Windows-native WHPX soak runner with per-test logs, JSON evidence, independent virtio-fs timeout, and residual-process checks
• preserve Windows virtiofs POSIX mode/UID/GID through Box stop, restart, and commit by updating the pinned libkrun runtime
• expand real smoke coverage for restart semantics, umask-derived modes, ownership, symlinks, and the full 2,048-file five-pass virtio-fs stress
• refresh deterministic Windows runtime/source archives, checksums, provenance, changelogs, and WHPX documentation

Depends on A3S-Lab/libkrun#5, stacked cleanly on A3S-Lab/libkrun#4.

Validation

• Windows WHPX soak: 10/10 passed in 851.181s; every sample exit code 0 and residual process count 0
• virtio-fs tar stress: 373.738s
• POSIX stop/restart/commit regression: 79.708s in the matrix and 80.03s standalone from the checked-in prebuilt DLL
• cargo test -p devices --target x86_64-pc-windows-msvc --lib -- windows::passthrough::tests --test-threads=1 (8 passed)
• cargo test -p a3s-libkrun-sys --lib (10 passed)
• cargo fmt --all -- --check
• scripts/package-libkrun-windows.ps1 -Check
• scripts/package-libkrun-source.ps1 -Check
• cargo package -p a3s-libkrun-sys --allow-dirty --no-verify (9,782,633-byte crate)

The deterministic krun.dll rebuild matched SHA-256 3af54645aa675356d631839ec64b863b1ef45ff1d83fb7530ffd39a83b1ea17a.