forgetest: the fixture's press docstring stops inviting a long press

Fixture._press said that a check that must reach the machine while the
button is down asks for the longest press the box can make, and the
unit test's comment repeated the invitation. That contradicts the
design: no press is ever held, every reader of the button catches the
default press (an edge, or a poll of 100 ms or less), and the box
clamps any request into 20 to 500 ms. The docstring now says what `ms`
is (the box's own 200 ms when it is not said, clamped by the box) and
that no check needs a longer press. The `ms` plumbing stays: it
mirrors the box's API field, and the box clamps regardless. The test's
pass-through assertion stays as the client's contract that a request
reaches the box as asked; the clamp is the box's, held by the fixture
firmware's policy test.

Proof: forgetest-ci "Unit tests" (`python -B -m unittest discover -s
tests -v` in forgetest), on Ubuntu 24.04 with python 3.12: the
ClientTests of test_fixture.py, 6 tests, OK. A grep for "longest
press" and "while the button is down" over fixture.py and
test_fixture.py prints nothing. Text only, so no negative control
applies. pyflakes reports no undefined name.

Acceptance: none. fixture.py is a non-suite module outside the shared
judges, and tests/ are in no fingerprint; no press length or path
changes.

forgetest/tests/test_fixture.py

@@ -173,7 +173,7 @@ class ClientTests(unittest.TestCase):
         f.act("button", "press")
         self.assertEqual(self.dev.calls[-1], ("POST", "/button", {}))
         time.sleep(0.15)
-        # a check that must reach the machine while the button is down asks for the longest press
+        # a pulse length asked for goes to the box as asked; the clamp is the box's
         f.act("button", "press", ms=500)
         self.assertEqual(self.dev.calls[-1], ("POST", "/button", {"ms": 500}))
         time.sleep(0.15)                                             # the fake's pulse ends