test: preserve invalid cwd encoding

[hyfdev]

Summary

• cover native-invalid Unix bytes inside an explicit cwd used by relative_with
• verify both borrowed and owned cwd arguments preserve the exact bytes and return owned results
• record the new encoding/context interaction in the semantic test strategy

Why

Existing invalid-encoding tests covered target and base paths, but not cwd components consumed while resolving relative inputs. A controlled mutation that silently dropped every non-UTF-8 cwd component passed the complete default and cached_current_dir suites. It changed the correct raw result ../invalid-\x80/target into ../anchor/target.

The new test reaches that shared resolution stack through the public API and catches the mutation with an exact byte assertion.

Validation

• targeted test with default and cached_current_dir configurations
• complete default suite and doctests
• controlled mutation fails only after this contract is added
• cargo fmt --all --check and git diff --check
• adversarial review independently derived the path result and reproduced the mutation failure

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