fix: gate Unix-only test helpers on Windows

[ORESoftware]

Problem

The exact Ubuntu/macOS/Windows static-mise certification found existing portability defects in test-only Unix fixtures. All 10 static environment unit tests and all 4 real mise_environment_cli tests passed on Windows, but cargo clippy --all-targets -- -D warnings then exposed:

1. Unix-only imports/helpers in tests/develop_shell_edge_cases.rs compiled as unused/dead code on Windows.
2. Shell-script executable fixtures in tests/e2e.rs imported Unix permission APIs or depended on /bin/sh without a target guard.

Fix

Apply matching #[cfg(unix)] boundaries only to test code that already requires Unix behavior:

• BTreeMap, OsStr / OsString, assert_success, and stdout in develop_shell_edge_cases;
• the executable writer, shell-script bin fixture/consumer helper, copy-mode hoisted-bin test, and install-dir shell fixture in e2e.

No production code or runtime behavior changes:

• Unix continues compiling and running the complete shell and executable-bin matrices.
• Windows continues compiling and running the platform-neutral parser, security, installation, and environment tests.
• All-target Clippy no longer compiles Unix permission APIs on Windows or reports intentionally absent Unix-only helpers as dead code.

Validation completed

A native windows-2025 materialization run passed:

• cargo fmt --all --check;
• git diff --check;
• cargo test --locked --test develop_shell_edge_cases;
• cargo test --locked --test e2e --no-run; and
• cargo clippy --locked --all-targets -- -D warnings.

The temporary materializer has been removed. The PR now contains exactly two ordinary test files with nine additive #[cfg(unix)] attributes. Normal repository CI, development-shell, OCI, policy, hardening, formal review, and polyglot workflows are rerunning on the cleaned exact head f6fa80f3866754c6a36089bb3b44d62ebee636b9.

Dependency chain

Found by: zed-pkg-test/zed-pkg-e2e#7
Static CLI candidate: #91
Linear: DEN-1439, DEN-1449

[linear-code]

DEN-1607