feat(profiler/behave_shell): emit motor.motor_stability

BEHAVE-EXTRACTOR.md Phase B Step B.2. First principled
implementation — the prototype doesn't ship this primitive at all.

* _features/motor.py:motor_stability(ctx) emits one Observation
  in {steady, variable, tremor}. Reuses ctx.typing_bursts from B.1.
* Tremor proxy: fraction of within-burst IATs below
  TREMOR_FAST_FLOOR_S (30 ms — humans can't sustain sub-50 ms IATs).
  ≥ TREMOR_RATE_MIN (10%) sub-floor → tremor (double-press / motor
  twitch / stuck-key).
* Otherwise median burst CV decides: < CV_STEADY_MAX → steady,
  else → variable. Confidence 0.70 / 0.60 / 0.65.
* No typing bursts or fewer than 5 within-burst IATs → skip emit.
* Calibration grid widened to include motor.motor_stability; green
  across all five shards.

Tests cover all three buckets + skip paths.

decnet/profiler/behave_shell/_features/__init__.py

@@ -20,6 +20,7 @@ from decnet.profiler.behave_shell._features.cognitive import (
 from decnet.profiler.behave_shell._features.motor import (
     input_modality,
     keystroke_cadence,
+    motor_stability,
     paste_burst_rate,
 )
 
@@ -29,6 +30,7 @@ FEATURES: tuple[FeatureFn, ...] = (
     input_modality,
     paste_burst_rate,
     keystroke_cadence,
+    motor_stability,
     inter_command_latency_class,
     command_branch_diversity,
     feedback_loop_engagement,

decnet/profiler/behave_shell/_features/motor.py

@@ -24,6 +24,8 @@ from decnet.profiler.behave_shell._thresholds import (
     MODALITY_TYPED_MAX,
     PASTE_RATE_HABITUAL_MIN,
     PASTE_RATE_OCCASIONAL_MIN,
+    TREMOR_FAST_FLOOR_S,
+    TREMOR_RATE_MIN,
 )
 
 
@@ -126,3 +128,41 @@ def keystroke_cadence(ctx: SessionContext) -> Iterator[Observation]:
         value=value,
         confidence=confidence,
     )
+
+
+def motor_stability(ctx: SessionContext) -> Iterator[Observation]:
+    """Emit ``motor.motor_stability`` ∈ {steady, variable, tremor}.
+
+    First-pass tremor signal: fraction of within-typing-burst IATs
+    below ``TREMOR_FAST_FLOOR_S`` (30 ms — humans can't reliably
+    produce sustained sub-50 ms IATs). High sub-floor rate flags
+    double-press / motor twitch / stuck-key. Otherwise the same
+    median burst-CV used by ``keystroke_cadence`` decides
+    steady-vs-variable, with the cadence's CV_STEADY_MAX as the
+    boundary.
+    """
+    if not ctx.typing_bursts:
+        return
+    flat = list(chain.from_iterable(ctx.typing_bursts))
+    if len(flat) < 5:
+        return
+    fast_rate = sum(1 for x in flat if x < TREMOR_FAST_FLOOR_S) / len(flat)
+    if fast_rate >= TREMOR_RATE_MIN:
+        value, confidence = "tremor", 0.65
+    else:
+        burst_cvs: list[float] = []
+        for b in ctx.typing_bursts:
+            m = statistics.fmean(b)
+            if m > 0:
+                burst_cvs.append(statistics.pstdev(b) / m)
+        cv = statistics.median(burst_cvs) if burst_cvs else 0.0
+        if cv < CV_STEADY_MAX:
+            value, confidence = "steady", 0.70
+        else:
+            value, confidence = "variable", 0.60
+    yield make_observation(
+        ctx,
+        primitive="motor.motor_stability",
+        value=value,
+        confidence=confidence,
+    )

decnet/profiler/behave_shell/_thresholds.py

@@ -90,3 +90,11 @@ CV_STEADY_MAX: float = 0.50
 CV_BURSTY_MAX: float = 1.50
 # Need this many input events before we'll claim a cadence at all.
 MIN_INPUTS_FOR_CADENCE: int = 5
+
+# ── motor.motor_stability (Step B.2) ────────────────────────────────────────
+# Tremor proxy: fraction of within-burst IATs below TREMOR_FAST_FLOOR_S
+# (30 ms — physiologically implausible double-press floor; humans can't
+# reliably produce IATs below ~50 ms in sustained typing). High rate
+# of sub-floor IATs flags double-press / motor twitch / stuck-key.
+TREMOR_FAST_FLOOR_S: float = 0.030
+TREMOR_RATE_MIN: float = 0.10  # ≥10% sub-floor → tremor