[codex] complete generated adapter IR milestone

[cssbruno]

Summary

Completes roadmap milestone 8 by making backend adapter generation use typed appgen IR as the shared source of truth for backend route metadata and backend handler emission across one-binary, split frontend proxy, and backend-only generated apps.

Changes

• Adds milestone 8 spec and implementation plan artifacts under docs/engineering/.
• Extends BackendAdapterIR with endpoint registration metadata, action/API/fragment adapter operation records, guard metadata, backend import metadata, dynamic route flags, decoders, handler calls, responses, and fallback metadata.
• Routes backend imports, route presence, CSRF/rate-limit/guard checks, split proxy matching, backend-only route registration, custom endpoint error pages, and action/API/fragment handler emitters through adapter IR.
• Adds appgen coverage for adapter metadata and split frontend proxy route matching.
• Updates roadmap and architecture docs to mark generated adapter IR implemented.

Verification

• go test ./internal/appgen
• go test ./internal/compiler ./internal/buildgen ./internal/appgen
• go build ./cmd/gowdk
• go run ./cmd/gowdk build --out /tmp/gowdk-m8-build --app /tmp/gowdk-m8-app examples/pages/*.gwdk
• go test ./...
• scripts/test-go-modules.sh