Project/bootstrap PHPUnit path still writes cache into readonly plugin source

[chubes4]

Summary

Merged #1799/#1804/#1805 at b8c4cfe passes WP Codebox’s new readonly-cache integration but still writes .phpunit.result.cache into a readonly plugin source under Homeboy Extensions’ real DB-activation canary.

Reproduction

1. Build WP Codebox at b8c4cfe.
2. From Default WP Codebox PHPUnit mounts to readonly Extra-Chill/homeboy-extensions#2257 run wordpress/scripts/test/playground-db-activation-smoke.sh with HOMEBOY_WP_CODEBOX_BIN and HOMEBOY_WP_CODEBOX_CORE_MODULE pointing to that build.
3. Observe PHPUnit passes 3 tests / 7 assertions, then the source-hash guard finds wordpress/tests/fixtures/test-db-activation-host/.phpunit.result.cache.

The generated recipe was debug-confirmed to declare the component mount readonly. WP Codebox’s own playground-phpunit-readonly-cache.integration.test.ts passes, so its fixture misses a bootstrap/config path used by the downstream runner.

Expected

All wordpress.phpunit bootstrap modes use the internal sandbox-private cache allocation and never write cache state into readonly plugin source. Add the Homeboy Extensions DB-activation fixture/contract as regression evidence.

[chubes4]

Resolved as stale runtime selection, not a WP Codebox defect. Forcing HOMEBOY_WP_CODEBOX_INSTALL_DIR to an empty path selected merged b8c4cfe; the real downstream canary passed 3 tests/7 assertions with no host cache and unchanged source hash. Remaining downstream runtime-selection/stale-artifact hardening is tracked in Extra-Chill/homeboy-extensions#2260.