R15C: tolerate legacy preflights in broker orphan scan

[bitmaster162]

Summary

• skip pre-broker legacy preflight rows that do not carry broker request identity during orphan adoption scan
• preserve fail-closed validation for modern matching broker candidates, malformed candidates, duplicate orphans, tamper, and existing registry mappings
• add regression coverage for legacy compatibility and matching malformed candidate rejection

Evidence

• base: 1f3cc0f147c96a3dc7630083dac663c91bf0b155
• head: 14a1b7cbdec95dd1c46b222f8f672b91aeb34665
• tree: 6fb01c65c1d777438bfa68066ec23017bcbb8cd6
• patch SHA-256: 2ff25d32148ce72638db791dd8bb5e79645a85fdf21bf769a9dc91f279d6c14b
• patch bytes: 3640
• GitHub compare: 1 commit ahead, 0 behind, 2 files
• GateBroker: 17 passed
• GateBroker + MCP broker + R14 + R15 + R15B: 158 passed, 1 skipped
• git diff --check: PASS
• py_compile: PASS

Provider CI

• required ubuntu / Python 3.11: success
• required windows / Python 3.11: success
• Analyze Python: success
• offline-shadow-continuity: success
• Advanced Security CodeQL: queued at Ready transition; not a required master status check
• reviews: 0
• review threads: 0

Runtime boundary

No live governance or TPM state is modified by this PR. Live GEN1 remained unchanged during validation:

• ledger f6cd175b627430047125fe8ed31cdb113f6bf6c14aa38dd04f8f5c6aa2f4bcfe
• registry 95b485225cb510749627efbfdab769ad7dc649b914ab6b7d60578f629e4fcfbf
• witness cd44b7d2e763b6a4095291912f8e3137aae53536daa76d36a06925abe755c8d7
• controller profile d732c991382a2edb99b9dcb39063224c2a62df2c56813668e76a74444c2965d4
• custody 49332781cffda2e7aec3a5ab7d41f4f0cd7b880ba2118b2e29492fb049917b98

Ready for review. No merge, deploy, reboot, or TPM/NV mutation authorized by this transition.