XY-1272: Close PostgreSQL configured-principal and ACL authority

[acgxv]

Summary

• close PostgreSQL 18 configured-principal, membership, owner, ACL, and migration-ledger authority against landed V8
• pin startup-packet search path and keep readiness/doctor fail-closed
• cover ledger column ACL drift and failure-safe dynamic credential-canary logging without serializing secret values
• align normative vNext authority and gate documentation

Validation

• full isolated PostgreSQL 18 harness passed, including restart, dump/restore, hostile search path, ledger-column ACL, forced secret-log failure, and doctor/readiness negatives
• cargo nextest run -p decodex-postgres -p decodex-runtime --all-targets --all-features: 135 passed
• SCCACHE_DISABLE=1 DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer cargo make check: 398 passed, 24 skipped
• architecture: 19 passed; gate/vstyle contract: 10 passed; CLI diagnostics: 3 passed
• fresh visible reviewer 019f6c55-43af-77c3-8531-016b2bd490c2: APPROVED/READY, no P0-P3 findings
• exact reviewed commit: ecbb0fbf5ce53e11f709d61866dcc1a02b21a71c

Linear: XY-1272