…ck()

Add a keyword-only `max_threads` argument to `dump_traceback()` and
`dump_traceback_later()`, defaulting to 100 to preserve existing
behavior. Allows server processes with many worker threads to dump
beyond the historical 100-thread cap (previously a hardcoded
`MAX_NTHREADS = 100` in `Python/traceback.c`).

The cap matters in practice: tstates are prepended to the
PyInterpreterState linked list, so the dump walks newest-first. With
more than 100 threads alive, the main thread (oldest, at the tail) is
silently elided from watchdog dumps -- exactly the thread that's
usually wanted.

The hardcoded value is moved to a new internal macro
`_Py_TRACEBACK_MAX_NTHREADS` in `pycore_traceback.h` so the in-tree
fatal-signal callers all reference one source of truth.

- Drop _Py_TRACEBACK_MAX_NTHREADS macro; use 0 as sentinel for default
  100 inside _Py_DumpTracebackThreads so internal callers don't have to
  pass the default explicitly.
- Rename max_nthreads -> max_threads everywhere for naming consistency
  with the public Python kwarg.
- Add max_threads kwarg to faulthandler.enable(); store in
  fatal_error.max_threads and pass through faulthandler_dump_traceback
  to the fatal-signal dump path on both POSIX and Windows.
- Drop the three redundant explanatory comments vstinner flagged.
- Doc: tighten the limitations bullet, drop implementation-detail
  mentions of the "..." truncation marker, switch versionchanged
  directives to "next", document the new enable() kwarg.
- Tests: assertEqual exact count, check whole-line "\n...\n" marker,
  use script_helper.assert_python_ok, drop the default-value test,
  add test_enable_max_threads exercising the fatal-signal path.
- NEWS: trim to two lines, mention all three functions.

The fatal-signal handler only dumps the current thread when the GIL
is disabled (pythongh-104812 / 3.14 versionchanged), so the truncation
marker assertion in test_enable_max_threads fails on free-threading
CI runs. Skip it there.

- Add a 'faulthandler' entry to Doc/whatsnew/3.15.rst (per
  @ZeroIntensity).
- Switch the max_threads parameters/fields and the nthreads loop
  counter in _Py_DumpTracebackThreads to Py_ssize_t (per @vstinner).

Per @vstinner's follow-up: also wire the kwarg through register() /
faulthandler_user. Stored in the per-signal user_signal_t struct; the
user-signal handler now passes user->max_threads to
faulthandler_dump_traceback instead of 0.

Doc, whatsnew, and NEWS entries updated to list all four functions.

- Use signal.raise_signal() instead of os.kill(os.getpid(), ...).
- Match test_dump_traceback_max_threads thread-cleanup pattern:
  keep refs, no daemon=True, set the stop event in a finally block,
  and join() the workers.