Add DL-06 offline VRRP transition contract

[Robinlee0929]

Summary:

• establishes the DL-06 transition-observer specification;
• adds the accepted offline dual-lab-vrrp-transition.v1 contract;
• records DL-06-01 review and Owner acceptance;
• preserves the existing Stage-2 and Dual-Lab live boundaries.

Validation:

• focused DL-06-01: 198 passed;
• Dual-Lab: 850 passed;
• Stage-2: 1,892 passed;
• accepted full pytest evidence: 4,870 passed, 0 skipped, 0 failed, 0 errors;
• publication-readiness review: PASS;
• application test results are retained accepted evidence, not reruns by this publication task; fresh PR Safe CI is separate.

Safety:

• offline transition contract only;
• no live I/O;
• no retry;
• no authorization reuse;
• no runtime observer loop;
• no configuration mutation;
• no provider/model integration;
• no automatic failover;
• DL-06-02 remains unauthorized;
• DL-06-03 remains blocked because hard 30-second whole-session deadline propagation/enforcement is not proven.

Retained non-blocking observations:

• 12 existing npm-reported dependency vulnerabilities;
• four redundant test parametrizations;
• oversize-test precision observation.

These findings remain open and are not claimed remediated. The dependency finding is not a general harmlessness determination.