fix(sandbox): support readonly Windows host mounts

[winrey]

Summary

• materialize read-only host directory mounts on Windows with directory junctions
• keep canonical mount sources in the sandbox read-only ACL roots
• reject reparse points in the workspace and mount namespaces
• preserve same-source alias reuse and conflicting-target rejection

Root cause

The sandbox runner only enabled host directory mount materialization on macOS. Windows already enforces read-only roots through the restricted sandbox token and ACL plan, but it had no safe alias from the sandbox mount namespace to the host directory. Every Windows workspace tool call therefore failed with MOUNT_PLATFORM_UNSUPPORTED.

Validation

• cargo check -p ahandd --all-targets
• mount tests: 11/11
• runner tests: 10/10
• Windows ACL plan tests: 3/3
• Windows readonly-root test: 1/1
• cargo test -p ahandd --lib: 430/431; the sole failure is a pre-existing localized Windows OS error-string assertion in config::load_tilde_tests::load_returns_err_for_missing_file
• cargo fmt --check
• git diff --check

Clippy was unavailable because the local stable toolchain does not have the component installed.