fix(smoke): isolate parallel tsx transforms

[chubes4]

Summary

• disable tsx's shared disk transform cache for parallel discovered TypeScript tests
• preserve existing cached behavior for declared and serial commands
• add a focused discovery contract for TypeScript, PHP, and serial command shapes

Evidence

PR #2455 run https://github.com/Automattic/wp-codebox/actions/runs/33534543967 failed in the parallel phase with a missing named export that is present in source. Focused, package, and contract runs pass, and no parallel test mutates the affected source. The bare tsx workers shared the only remaining cross-process state: tsx's global per-user transform cache.

Verification

• npx tsx tests/smoke-discovery.test.ts
• npm run build
• npx -p node@22 node node_modules/tsx/dist/cli.mjs scripts/run-smoke.ts --group=check (343 commands, concurrency 8, Node 22.23.2)

Fixes #2456.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode analyzed the CI-only loader failure, audited shared repository mutation and tsx cache boundaries, implemented the parallel-worker isolation, and ran the exact Node 22 aggregate. Chris Huber directed and owns the work.