Configure Playground runtime request workers

[chubes4]

Fixes #2087.

What changed

• Adds backend-neutral runtime.workers support for integers from 1 through 64 or auto.
• Propagates the option through PHPUnit recipe building, dry-run plans, runtime normalization, and recipe execution.
• Passes the selected count to Playground CLI while preserving the existing default of six when omitted.
• Rejects invalid builder input instead of silently dropping it.

Why

Playground applies mounted resources to every request worker. Large generated runtimes can therefore multiply startup work by six even when the workload needs only one request worker. Bounded execution concurrency remains an independent orchestration concern.

Verification

• npm run build
• npm run test:runtime-workers
• npm run test:runtime-neutral-contracts
• npm run test:recipe-runtime-backend-normalization
• npm exec -- tsx tests/playground-cli-runner-bootstrap-ini.test.ts
• git diff --check

The combined test:runtime-services command reaches an existing macOS native-MariaDB containment failure after its generic and external-MySQL portions pass; this change does not touch that provider.

AI assistance

OpenCode using openai/gpt-5.6-sol implemented the runtime worker-count contract and tests after diagnosing the large-mount startup scaling failure. Chris Huber reviewed and owns the submitted change.