Sync trust-file date stamps for burn-in retirement

PR #1828 changed the CI-guard language in AUDIT.md, AXIOMS.md, and
TRUST_ASSUMPTIONS.md but did not bump the Last Updated stamps. The
synchronization non-negotiable (CLAUDE.md #1) covers date stamps for
any CI-boundary change, so refresh them to 2026-05-12.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Th0rgal

AUDIT.md

@@ -67,4 +67,4 @@ theorem is proved.
    command.
 5. Run `make check`; run targeted Lean builds for changed proof modules.
 
-**Last Updated**: 2026-04-21
+**Last Updated**: 2026-05-12

AXIOMS.md

@@ -204,4 +204,4 @@ Any commit that adds, removes, renames, or moves an axiom must update this file
 
 If this file is stale, trust analysis is stale.
 
-**Last Updated**: 2026-04-09
+**Last Updated**: 2026-05-12

TRUST_ASSUMPTIONS.md

@@ -113,5 +113,5 @@ High-level semantics can expose intermediate state in reverted computations. EVM
 
 ---
 
-**Last Updated**: 2026-04-20
+**Last Updated**: 2026-05-12
 **Maintainer Rule**: Update on every trust-boundary-relevant code change.