Declare isolated workers in bounded recipe integration

[chubes4]

Problem

tests/bounded-recipe-plan.integration.test.ts requests bounded-plan concurrency 2 but creates a WordPress Playground runtime without a worker count. Since #2323, mutable Playground runtimes intentionally default to one request worker. Both concurrent wordpress.run-php entries therefore occupy a one-worker transport and time out after 30 seconds.

This remained hidden because #2459 corrected only the fake-runtime unit concurrency proof. The real recipe integration still omitted the runtime capability it exercises. It failed both in the full aggregate and standalone while verifying #2464.

Desired outcome

Make the integration fixture explicitly request two Playground workers, matching its plan concurrency and per-entry process identities. Preserve the production default of one worker for mutable-state coherence.

Acceptance criteria

• The real bounded recipe integration executes both entries successfully.
• The fixture explicitly declares the worker concurrency it requires.
• Production runtime defaults remain unchanged.
• The full smoke aggregate passes.

AI assistance

GPT-5.6 Sol via OpenCode traced the integration timeout through the bounded scheduler and Playground request-worker configuration and drafted this tracker under Chris Huber's direction.