fix(security): keep boundary guard self-clean

[allenwoods]

Summary

Prevent the boundary guard and its tests from matching their own fixture literals.

• replace one explanatory literal with neutral wording;
• construct sensitive test fixtures at runtime, as the existing tests already do elsewhere;
• keep every positive/negative assertion and checker behavior unchanged.

Verification

• 53 complete boundary tests passed;
• both guard source files pass targeted content scanning;
• independent static review found no P0/P1/P2 and confirmed the runtime fixture bytes are unchanged;
• git diff --check passed.

Guard update exception

This is a dedicated two-file guard update. Its branch was pushed with the required audited --no-verify exception. The trusted guard-integrity step is expected to reject it until an explicit security-admin merge. Reinstall the immutable hook after merge.