Give unknown doctor results a distinct exit code

[myobie]

Summary

• return 0 only when every doctor finding is verified healthy
• return 1 when a problem or setup remains
• return 3 when the run has only unknown findings
• document all codes and reserve Clap code 2 for usage errors

Red proof

The new integration test failed before the implementation because an unknown finding returned 1. The help test also failed because doctor documented only codes 0 and 1.

Verification

• cargo test --lib: 471 passed, 4 ignored
• cargo test --bin fabric: 19 passed
• cargo test --test doctor: 2 passed
• git diff --check

The repository-wide format check has existing drift outside this patch. The new integration test passes its individual rustfmt check.

No release or deployment is part of this change.