test: preserve non-UTF-8 ambient cwd

[hyfdev]

Summary

• add a Linux-only child-process test that starts from a real cwd containing the invalid native byte 0x80
• require absolutize, try_absolutize, relative, and try_relative to preserve the cwd bytes exactly
• execute the contract under both default and cached_current_dir configurations through the native CI inventory
• record the ambient native-encoding coverage in the testing strategy

Why

Existing tests cover non-UTF-8 inputs with an ordinary ambient cwd and non-UTF-8 explicit cwd arguments, but not an ambient cwd whose own filesystem spelling is non-UTF-8.

A controlled mutation rebuilt cwd through to_string_lossy() in both feature branches. The existing complete default and cached suites remained green, even though the mutation changes the native byte 0x80 into the UTF-8 replacement sequence EF BF BD before all four ambient APIs use it.

The new test runs in a fresh integration-test child process so the cached cwd cannot be initialized by the parent. It derives expected paths from std::env::current_dir() and compares raw bytes, without imposing a Cow or allocation requirement on the non-UTF-8 case.

Platform scope

Linux supplies the executable filesystem case because macOS rejects the invalid-byte directory name with EILSEQ. The test is therefore required only in the native Linux inventory; explicit-cwd tests continue to cover native-invalid Unix composition independently of the host filesystem.

Validation

• complete default suite: pass
• complete cached_current_dir workspace suite: pass
• full Clippy with warnings denied: pass
• documentation and package verification: pass
• Linux library and integration-test metadata cross-compilation: pass
• controlled lossy-cwd mutation: previous default 112 and cached 113 tests remained green
• native Linux child-process test: pass under default and cached_current_dir
• native Linux required-test registration: pass
• all seven hosted checks: pass
• independent adversarial review: pass

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