Add experimental local celld runtime adapter #421

Intent

Add celld as an experimental sibling runtime in EdgeZero. Run genuine Rust WASM and the workers-rs shim directly in celld, reuse the shared Worker transport with explicit runtime policy, and provide registered local build/serve and JSON scaffolding.

Implementation: #420. Companion application: stackpop/mocktioneer#122.

Scope

• Explicit celld identity and provider metadata without borrowing Cloudflare guarantees.
• Bounded default-stream config reads through real KV and verified typed envelopes, while preserving Cloudflare BYOB defaults.
• Generated dependencies and ignores for build output, local runtime state and dev variables.
• Clear rejection of secret stores and unsupported remote operations.

This issue tracks the local adapter implementation. AWS deployment, managed secrets, remote config publication, fleet qualification and Trusted Server identity Add/CAS/generation semantics are outside its completion criteria. It does not close or replace Axum production-readiness work.

The implementation PR records the focused checks and real generated-consumer/Mocktioneer HTTP acceptance. Local success is not production qualification.