test(metadata): Windows source -> Linux destination ACL round-trip (WAS-7 #2312)

[oferchen]

Summary

• Adds crates/metadata/tests/windows_to_linux_acl_roundtrip.rs covering the receiver-side apply_xattrs_from_list dispatch for the reserved user.win32.security_descriptor xattr slot introduced by WAS-5 (PR feat(metadata): wire --acls to Windows DACL (#2310) #4388).
• Simulates a Windows source on a non-Windows host by hand-crafting an XattrEntry { name = "user.win32.security_descriptor", value = sample_sddl } and feeding it through metadata::apply_xattrs_from_list against a TempDir file.
• Sibling to the workspace-root tests/acl_windows_to_linux_roundtrip.rs, which pins the pure-mapping contract (DACL <-> POSIX bits + RsyncAcl::names). This file exercises the actual xattr application path.

Scenarios wired

• simulated_windows_xattr_dropped_on_linux (cfg unix): standard user.* slots survive verbatim. The reserved slot has a tolerant assertion that accepts either the WAS-5 strict-drop arm or the pre-WAS-5 verbatim-store fallback, so the test does not flake against in-flight master and tightens automatically once feat(metadata): wire --acls to Windows DACL (#2310) #4388 merges.
• simulated_windows_xattr_applied_on_windows (cfg target_os = "windows"): apply the reserved slot, then read the DACL back via metadata::read_dacl_sddl and assert the descriptor includes a DACL section.
• pure_posix_acl_roundtrip_unaffected_by_reserved_slot (cfg unix): standard user.* payload with no reserved slot; both standard slots are applied and the reserved name does not appear in the destination listing read back via read_xattrs_for_wire.

Test plan

• cargo fmt --all clean.
• Tests hermetic via tempdir().
• File gated on feature = "acl" and feature = "xattr"; excluded when either is off.
• Filesystem support pre-checked via a probe apply_xattrs_from_list write; skips gracefully on tmpfs / sandboxed CI volumes.

Closes #2312.