Add proptest property-based tests for NETSUKE_NINJA resolver invariants

[coderabbitai]

Background

The NETSUKE_NINJA feature (implemented in PR #323, closes #83) added example-based tests for the two common cases (unset env var, valid override). However, the resolver has an input-state invariant across the full space of env-var values (empty string, non-UTF-8, absolute path, bare name, path with spaces, etc.) that example-based tests do not cover.

Required work

Add property-based tests using proptest that:

1. Generate arbitrary valid and invalid values for NETSUKE_NINJA.
2. Assert that the resolved program (output of resolve_ninja_program / resolve_ninja_program_utf8) matches the value logged in the "Executing command:" trace line in all cases.
3. Assert that when NETSUKE_NINJA is unset or contains a non-UTF-8 / non-representable value the resolved program falls back to "ninja".
4. Target both src/runner/process/mod.rs (unit level) and a suitable binary-level test in tests/.

Acceptance criteria

• At least one proptest covering the resolver invariant: resolved program == logged program for all valid non-empty NETSUKE_NINJA values.
• At least one proptest covering the fallback invariant: invalid / absent NETSUKE_NINJA always resolves to "ninja".
• Tests pass under cargo test.
• No broad lint suppressions beyond narrowly scoped #[expect(...)] with reasons.

Backlinks

• PR: Document Ninja executable override (#83) #323
• Issue Support overriding Ninja executable via NETSUKE_NINJA #83
• Raised by: @leynos

[coderabbitai]

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