A bare 3.14 previously resolved to the freethreaded build only because it
was the sole final 3.14 in uv's managed cache. Use 3.14t to lock in
free-threading against future plain 3.14 installs.
ProcessPoolExecutor relied on private API, lost os.setpriority and
auto-restart. Revert to the subprocess worker but spawn the current
executable with --worker instead of a separate eris-worker console
script, keeping a single entry point for shiv/pyinstaller. Bump
.python-version to 3.14.
