Play Framework

feat(testrunner): parallelize unit tests across classes via VT pool

play auto-test was previously serial across every test class — FirePhoque
opened one /@tests/<class> URL, waited for the result HTML to appear on
disk, then moved to the next. With the framework now unconditional VT,
the server side handles concurrent test-class requests fine; the loop
itself was the bottleneck.

Stage 1 of the test-parallelism roadmap:

* TestRunner.list now tags entries with U:/F:/S: prefixes so FirePhoque
  can route by category. Unprefixed entries fall back to the serial
  path for graceful degradation.
* FirePhoque splits unit tests off to a virtual-thread-per-task executor
  capped by a Semaphore (default 16, override via
  -DunitTestParallelism=N). Each unit test fires through java.net.http.
  HttpClient (thread-safe; HtmlUnit's WebClient is not, and unit tests
  don't need browser semantics anyway). Result-file polling and pass/
  fail determination match the existing serial path.
* Functional + Selenium tests stay on the WebClient serial path —
  FunctionalTest's static savedCookies/renderArgs would race under
  parallelism (Stage 2 territory).
* Output prints in completion order under a shared lock so lines
  remain atomic across concurrent workers.
* AtomicBoolean replaces the bare boolean for cross-VT pass/fail
  aggregation.

Expected speedup: 5-15x on typical app suites where unit tests dominate.
Stages 2 and 3 (FunctionalTest ThreadLocal refactor, fully-parallel
FirePhoque) remain available as follow-ups.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>