backend: make schema reflection data-only

[turinglambdaai]

Summary

• replace accidental public backend registry structs and mutable State internals with the opaque state? boundary
• add one immutable, data-only backend-schema snapshot and migrate code generation to it
• document the pre-1.0 migration and architecture boundary; keep white-box concurrency access confined to tests

Advances #185.

Validation

• raco test tests/backend.rkt tests/codegen.rkt tests/state-order-lock-init.rkt tests/state-backpressure.rkt tests/state-cancel-backpressure.rkt tests/documentation.rkt — 429 tests passed
• raco test tests — 8,862 tests passed
• git diff --check