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,