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.

tests/profiler/behave_shell/test_environmental_numpad_usage.py

@@ -0,0 +1,57 @@
+"""Step F.5: ``environmental.numpad_usage``."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+PRIMITIVE = "environmental.numpad_usage"
+
+
+def _of(observations: list, primitive: str):
+    obs = [o for o in observations if o.primitive == primitive]
+    assert len(obs) == 1, f"expected exactly one {primitive}, got {len(obs)}"
+    return obs[0]
+
+
+def _typed(text: str, t0: float = 0.0, dt: float = 0.10) -> list[AsciinemaEvent]:
+    return [(t0 + i * dt, "i", c) for i, c in enumerate(text)]
+
+
+def test_below_min_typed_chars_no_emission() -> None:
+    out = list(extract_session(_typed("ls\r"), sid="np-tiny"))
+    assert [o for o in out if o.primitive == PRIMITIVE] == []
+
+
+def test_no_digit_runs_emits_not_detected() -> None:
+    """50+ typed chars, none of them digit runs → not_detected."""
+    text = "the quick brown fox jumps over the lazy dog repeatedly\r"
+    out = list(extract_session(_typed(text, dt=0.10), sid="np-text"))
+    obs = _of(out, PRIMITIVE)
+    assert obs.value == "not_detected"
+
+
+def test_slow_digit_typing_not_detected() -> None:
+    """Slow digit typing (typing-speed cadence) → not_detected."""
+    # 100ms IAT between digits — too slow for numpad
+    text = "1234567890" * 6 + "\r"  # 60 digits + return for command boundary
+    out = list(extract_session(_typed(text, dt=0.10), sid="np-slow"))
+    obs = _of(out, PRIMITIVE)
+    assert obs.value == "not_detected"
+
+
+def test_fast_digit_run_emits_detected() -> None:
+    """Sub-50ms digit cadence over a 4+ run → detected."""
+    # Build a session with 50+ chars first, then a fast digit burst
+    events: list[AsciinemaEvent] = []
+    # Filler typing (slow)
+    for i, c in enumerate("the quick brown fox jumps over the lazy dog filler"):
+        events.append((i * 0.10, "i", c))
+    # Fast digit run starting at t=10s — IAT=20ms
+    base = 10.0
+    for i, d in enumerate("1234567890"):
+        events.append((base + i * 0.020, "i", d))
+    events.append((20.0, "i", "\r"))
+    out = list(extract_session(events, sid="np-fast"))
+    obs = _of(out, PRIMITIVE)
+    assert obs.value == "detected"