Amend exact-microsecond quota authority

[acgxv]

Summary

• require exact, lossless UTC Unix-microsecond quota timestamps with no rounding or truncation
• define typed mutation schema /2 replay and the V8 structural zero-state migration boundary
• preserve rejected-candidate provenance and reclassify the next XY-1274 implementation as candidate 1 of the redesigned boundary

Validation

• DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer cargo make check
• architecture tests: 19/19
• gate contract tests: 10/10
• focused links: 21/21
• git diff --check and repository-wide semantic reverse scan
• independent exact-candidate review: APPROVED/READY for fingerprint 06bc2ebea8105bae2715ff92987b346fb4271da19c5882741b7ce5424f61945b

Tracking

Closes XY-1341 and unblocks the redesigned XY-1274 implementation. This authority amendment does not itself enable live quota routing.