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/_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