feat: wire the staging environment (control-stg remote + staging dispatch frame)

[MrCoder]

Summary

• Adds the control-stg remote (staging control Worker, fetch+compute) so the resolver's invokeRemote routing has a staging target — previously a staging install would silently route to the dev control key.
• Adds the staging dispatch Worker to permissions.external.frames so the staging Custom UI may embed the served mini-site iframe.
• Resolver derives control-stg from a -staging CONTROL_BASE_URL.

Staging infra stood up alongside (out-of-band, staging-scoped): mini-sites-staging dispatch namespace created (also un-breaks master's staging auto-deploy), both staging Workers deployed, K_GRANT/CONTROL_SHARED_SECRET/WFP_API_TOKEN_PROVISIONING set.

Test plan

• pnpm test 224/224 + typecheck green (no Worker code touched).
• Live staging spot check (Forge staging deploy + install on lite-stg): macro renders and the staging control Worker logs via:"fit".

🤖 Generated with Claude Code

https://claude.ai/code/session_01UDYKDJBU8KZP9SWMaxNxSp