Execute bounded PHP commands through clean Playground processes

## Problem

Bounded runtime entries with `processIdentity` require isolated PHP state while reusing one Playground server, its mounted filesystem, extensions, and runtime services.

The current HTTP request-worker approach does not provide that isolation: the request worker may already have loaded the managed WordPress installation. A project-owned PHPUnit bootstrap that loads its own WordPress tree then fails on inherited symbols such as `add_filter()`. Generic `proc_open()` spawning remains inside the same PHP-WASM environment and does not solve the collision.

Project bootstrap mode also currently prepends the managed `/wordpress/wp-load.php`, which violates project ownership, while PHPUnit configurations without a bootstrap are rejected even though native PHPUnit accepts them.

## Upstream dependency

WordPress Playground issue: https://github.com/WordPress/wordpress-playground/issues/4145
WordPress Playground PR: https://github.com/WordPress/wordpress-playground/pull/4146

The Playground PR exposes `runInFreshProcess()` with preserved mounts/configuration and collision-free concurrent code execution.

## Required changes

- Route bounded code executions with a process identity through `playground.runInFreshProcess()`.
- Remove the HTTP request-worker endpoint and payload-file transport.
- Preserve per-entry runtime environment injection, including `bootstrap=none`.
- Let project bootstrap mode own WordPress loading instead of prepending managed `wp-load.php`.
- Permit project-mode PHPUnit configurations that declare no bootstrap.
- Fail clearly when the selected Playground backend lacks clean-process support.

## Acceptance criteria

- A project bootstrap may define `add_filter()` without colliding with managed WordPress symbols.
- Concurrent bounded entries retain isolated PHP state and their own service environment.
- Existing non-bounded Playground commands retain current behavior.
- Focused runner/bootstrap tests pass.
- Downstream TeamCity-equivalent validation passes all 680 PHPUnit suites with zero failures, timeouts, or cancellations.

Start with the bounded execution path and the playground.runInFreshProcess() entry point, then review the runner/bootstrap tests for environment injection, project-owned WordPress loading, and bootstrap=none. Done means clean-process support fails clearly when unavailable, concurrent bounded entries remain isolated, non-bounded commands are unchanged, focused tests pass, and the 680-suite validation has no failures, timeouts, or cancellations.

Written by the indexing model from the issue text.