fix: --glitch-at without --glitch-width now does something rather than silently nothing

--glitch-width defaulted to 0.0, so --glitch-at alone produced no glitch and no
warning — the same class of silent no-op that cost a debugging cycle earlier on this
branch.  The default is now 0.002 s (2 ms), matching the standalone probe run.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/mock_udpstreamer.py

@@ -222,8 +222,9 @@ def main():
     ap.add_argument(
         "--glitch-width",
         type=float,
-        default=0.0,
+        default=0.002,
-        help="width of each injected pulse, in stream seconds",
+        help="width of each injected pulse, in stream seconds (default 0.002 s; a zero width "
+        "silently disables the glitch even when --glitch-at is set)",
     )
     ap.add_argument(
         "--quiet-at",