agents-sub-agent-id-ioctx repro fixture

Endpoints:
  GET /agents/<facetName>
    - this.subAgent(ChildAgent, name) then child.getName()
      → partyserver Server.name getter → this.ctx.id.name
  GET /raw/<facetName>
    - this.subAgent(...) then read this.ctx.id.name directly
      inside the facet RPC handler.
  GET /faulted/<facetName>
    - synthetic fault-injection probe (Proxy on ctx.id that
      throws the production cross-IoContext error). Useful
      for verifying any defensive partyserver / agents fix
      locally.

Empirical baseline (so far):
  - vite dev:                 /agents, /raw -> 200 (expected)
                              /faulted     -> 500
  - wrangler dev:             same as vite dev
  - wrangler deploy:          ?  awaiting confirmation —
                              the partyserver-only fixture
                              also returns 200 in production,
                              so this fixture is the next
                              bisection step toward the
                              cloudflare/agents#1410 trigger.