[codex] Ensure stateful builds use deployment target

[arjunkomath]

Summary

• Route stateful service builds to the single active replica server instead of the locked server or any same-arch build server.
• Resolve stateful target platforms from that active deployment target and fail early when the target server is missing, offline, or missing architecture metadata.
• Keep stateless multi-arch build assignment behavior, including allowed build server filtering.

Why

Stateful services should build on the same machine that will deploy/run them. The previous path could fall back to generic architecture matching when a stateful service was not yet locked, which could build on a different machine from the deployment target.

Validation

• pnpm test
• pnpm exec tsc --noEmit
• git diff --check
• Global Smart Review pass outside sandbox, with review feedback addressed in tests and helper cleanup.