fix(dsl): confine mutation path variants

[rustytrees]

Description

Pathname and FileUtils mutation variants now use resolved boundary checks instead of lexical prefixes. The checks confine symlink destinations and targets for scalar, array, and hash forms.

The regression tests cover mkpath, unlink, install_symlink, ln_sf, and rm, including intermediate directory symlinks and outside targets.

Related Issue

Closes #855.

Notes for Reviewers

Verification completed:

• Focused DSL builtins: 72 passed
• zig build test-one: 2,424 passed
• zig build test: 5,067 passed, 3 skipped
• Commit signature verified with git verify-commit

[indaco]

@rustytrees Thanks for the contribution! One small request: could you please instruct your agent to follow the PR template provided in the repository for PRs? It helps keep the PRs consistent and easier to review. Thanks!

[rustytrees]

I published a runnable PoC for the Pathname.mkpath confinement failure fixed here:

• h03-mkpath source
• runner at signed commit bd0e2a4
• digest-pinned release

git clone https://github.com/rustytrees/malt-security-pocs.git
cd malt-security-pocs
git checkout bd0e2a4d088cc63c8a7ab1613e851d23808b5a4d
./scripts/run-poc.sh --malt /path/to/malt --expect vulnerable h03

The runner uses malt migrate with an isolated synthetic Homebrew keg because Malt's tap installer does not execute def post_install. Migration reaches executeDslPostInstallFields, the native interpreter, and Pathname.mkpath. On exact efd2b99, the formula created /tmp/malt-security-poc-h03/created outside MALT_PREFIX. With the combined patched build containing this PR, migration completed and installed the package, but the outside directory remained absent.

This PoC dynamically covers Pathname.mkpath. The regression tests in this PR provide the broader coverage for Pathname.unlink, Pathname.install_symlink, FileUtils.ln_sf, FileUtils.rm, and the alternate argument forms.