feat(profiler/behave_shell): emit motor.keystroke_cadence

BEHAVE-EXTRACTOR.md Phase B Step B.1.

* SessionContext gains typing_bursts: tuple[tuple[float, ...], ...]
  built by _split_typing_bursts(iats) — splits at gaps > IKI_THINK_MAX_S
  (1.5s) and drops bursts of fewer than 3 IATs. Mirrors prototype's
  _split_into_bursts at BEHAVE/prototype_extractors/shell/extract.py:275.
* _features/motor.py:keystroke_cadence(ctx) emits one Observation
  in {steady, bursty, hunt_and_peck, machine}. Median CV across
  typing bursts; mean IKI < IKI_MACHINE_MAX_S paired with CV <
  CV_MACHINE_MAX → machine. Confidence 0.85/0.70/0.65/0.60 per the
  prototype's calibration history.
* < MIN_INPUTS_FOR_CADENCE inputs or zero typing bursts → skip
  emission. v0.1 emits only the burst-CV variant; the prototype's
  NAIVE session-CV variant is parked for v0.2.
* Calibration grid widened (PHASE_A_PRIMITIVES → PHASE_AB_PRIMITIVES)
  to include motor.keystroke_cadence. Grid green across all five
  shards.

Tests: too-few-inputs → no emit, all-think-pauses → no burst → no
emit, uniform IATs → steady, sub-5ms → machine, mixed-pace → bursty,
extreme bimodal → hunt_and_peck.

decnet/profiler/behave_shell/_thresholds.py

@@ -75,3 +75,18 @@ FEEDBACK_MIN_PAIRS: int = 5
 # via Hartigan dip is filed for v0.2).
 PAUSE_CV_METRONOMIC_MAX: float = 0.40
 PAUSE_CV_BIMODAL_MIN: float = 1.50
+
+# ── motor.keystroke_cadence (Step B.1) ──────────────────────────────────────
+# Typing bursts split at gaps > IKI_THINK_MAX_S so think-pauses between
+# commands don't inflate the within-burst CV. Mirrors the prototype's
+# _split_into_bursts (BEHAVE/prototype_extractors/shell/extract.py:275-286).
+IKI_THINK_MAX_S: float = 1.50
+# Sub-human floor for the "machine" classification — only paired with a
+# pathologically uniform CV, since real humans never produce sub-5ms IATs
+# in a sustained burst.
+IKI_MACHINE_MAX_S: float = 0.005
+CV_MACHINE_MAX: float = 0.05
+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