Enforce Flex mapping drift checks

[Jason Morse (JasonVMo)]

Summary

• add an offline consistency gate for the Flex YAML mapping, Theme projection, non-Fluent defaults, and declared token types
• run the consistency gate before design codegen and cover each invariant with focused Jest diagnostics
• pin the x3 source revision and exact snapshots with a network-optional, offline-verifiable drift report and documented update workflow

Validation

• yarn build
• yarn lage test
• yarn lage lint
• yarn lint-repo
• yarn check-publishing
• yarn change:check

Fixes #4266