Build the test suite on Windows by gating Unix-only tests

[Tyagiquamar]

Problem

cargo test fails to compile on Windows. Three files use Unix-only APIs unconditionally:

• src/spec_identity.rs — the unit-test module imports std::os::unix::fs and runs two symlink-escape tests (unix_fs::symlink).
• tests/spec_identity_tests.rs — same import plus two symlink tests.
• tests/adapter_tests.rs — imports std::os::unix::fs::PermissionsExt (for set_mode(0o755) on fake host shims) in three dogfood tests that also execute bash scripts.

On Windows the build stops at error[E0432]: unresolved import std::os::unix, so no test binary builds at all — including all platform-independent tests.

Provenance: found by running cargo test on Windows; the repo's local CI gate has only been exercised on Unix so far. No matching issue exists.

Fix

Gate exactly the Unix-dependent imports, helpers, and tests behind #[cfg(unix)]:

• The two symlink tests per file depend on unix_fs::symlink; the three adapter dogfood tests create executable bash shims via PermissionsExt. All are inherently POSIX-only.
• Shared non-Unix helper machinery that becomes unused when those tests are gated out (HOST_MEDIATED_DOGFOOD, host_mediated_dogfood_guard, json/fs/Path/tempdir imports) is gated with them, so both platforms compile warning-free.
• Everything else — 108 lib unit tests and all platform-independent integration tests — now builds and runs on Windows unchanged. On Unix, compilation is byte-for-byte identical (#[cfg(unix)] is a no-op there).

Testing

Windows (x86_64-pc-windows-gnu, Rust 1.98):

• Before: error[E0432]: unresolved import std::os::unix — no test binaries built.
• After: cargo test → all green: 108 unit + 65 integration tests pass, 0 failed, no warnings.
• cargo fmt --check clean; cargo clippy --all-targets --all-features -- -D warnings clean.