feat(profiler/behave_shell): emit environmental.numpad_usage

Sliding-window scan over single-char digit input events. A run of
NUMPAD_RUN_MIN (4) consecutive digit events whose pairwise IATs are
all ≤ NUMPAD_FAST_IAT_S (50ms) → detected. Otherwise → not_detected.
Skips below NUMPAD_MIN_TYPED_CHARS (50) typed chars. Confidence cap
0.50 per the registry's weak-signal flag.

decnet/profiler/behave_shell/_features/__init__.py

@@ -27,6 +27,7 @@ from decnet.profiler.behave_shell._features.cognitive import (
 from decnet.profiler.behave_shell._features.environmental import (
     keyboard_layout,
     locale,
+    numpad_usage,
     shell_type,
     terminal_multiplexer,
 )
@@ -77,4 +78,5 @@ FEATURES: tuple[FeatureFn, ...] = (
     terminal_multiplexer,
     locale,
     keyboard_layout,
+    numpad_usage,
 )