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.

tests/profiler/behave_shell/test_calibration_grid.py

@@ -31,13 +31,16 @@ from decnet.profiler.behave_shell import extract_session
 from decnet.profiler.behave_shell._parse import parse_shard_line
 
 
-PHASE_A_PRIMITIVES: frozenset[str] = frozenset({
+PHASE_AB_PRIMITIVES: frozenset[str] = frozenset({
+    # Phase A — calibration floor
     "motor.input_modality",
     "motor.paste_burst_rate",
     "cognitive.inter_command_latency_class",
     "cognitive.command_branch_diversity",
     "cognitive.feedback_loop_engagement",
     "cognitive.inter_command_consistency",
+    # Phase B — motor.* completion (lands one primitive per commit)
+    "motor.keystroke_cadence",
 })
 
 
@@ -105,7 +108,7 @@ def test_shard_emits_all_phase_a_primitives(
     obs = _all_observations(path)
     assert obs, f"{class_label}: extractor produced zero observations"
     seen = {o.primitive for o in obs}
-    missing = PHASE_A_PRIMITIVES - seen
+    missing = PHASE_AB_PRIMITIVES - seen
     assert not missing, (
         f"{class_label} ({shard_file}) missing primitives: "
         f"{sorted(missing)}"
@@ -142,7 +145,7 @@ def test_shards_are_discriminative_across_classes(
     # At least one primitive should produce different majority values
     # across the present classes.
     discriminative_primitives: list[str] = []
-    for prim in PHASE_A_PRIMITIVES:
+    for prim in PHASE_AB_PRIMITIVES:
         values = {by_class[c].get(prim) for c in by_class if prim in by_class[c]}
         if len(values) >= 2:
             discriminative_primitives.append(prim)

tests/profiler/behave_shell/test_motor_keystroke_cadence.py

@@ -0,0 +1,85 @@
+"""Step B.1: ``motor.keystroke_cadence``."""
+from __future__ import annotations
+
+import random
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+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_events(iats: list[float], terminator: bool = True) -> list[AsciinemaEvent]:
+    """Build a typed input stream where consecutive single-char events are
+    separated by ``iats``."""
+    events: list[AsciinemaEvent] = []
+    t = 0.0
+    events.append((t, "i", "a"))
+    for x in iats:
+        t += x
+        events.append((t, "i", "b"))
+    if terminator:
+        events.append((t + 0.1, "i", "\r"))
+    return events
+
+
+def test_too_few_inputs_no_emission() -> None:
+    out = list(extract_session(_typed_events([0.1, 0.1]), sid="cad-low"))
+    assert [o for o in out if o.primitive == "motor.keystroke_cadence"] == []
+
+
+def test_huge_think_pauses_yield_no_typing_bursts() -> None:
+    # Two events 5s apart → no IAT under IKI_THINK_MAX_S, and only 1
+    # IAT total — below the 3-IAT-per-burst minimum. No burst, no emit.
+    events: list[AsciinemaEvent] = [
+        (0.0, "i", "a"),
+        (5.0, "i", "b"),
+        (10.0, "i", "c"),
+        (15.0, "i", "d"),
+        (20.0, "i", "e"),
+    ]
+    out = list(extract_session(events, sid="cad-no-bursts"))
+    assert [o for o in out if o.primitive == "motor.keystroke_cadence"] == []
+
+
+def test_uniform_iats_emit_steady() -> None:
+    iats = [0.15] * 12
+    out = list(extract_session(_typed_events(iats), sid="cad-steady"))
+    obs = _of(out, "motor.keystroke_cadence")
+    assert obs.value == "steady"
+    assert obs.confidence == 0.70
+
+
+def test_machine_iats_emit_machine() -> None:
+    # Sub-5ms IATs with near-zero CV — no terminator IAT to inflate the
+    # variance away from machine
+    iats = [0.002] * 20
+    out = list(extract_session(_typed_events(iats, terminator=False), sid="cad-machine"))
+    obs = _of(out, "motor.keystroke_cadence")
+    assert obs.value == "machine"
+    assert obs.confidence == 0.85
+
+
+def test_bursty_iats_emit_bursty() -> None:
+    # Mean ~0.15 with moderate variance, CV between 0.5 and 1.5
+    rng = random.Random(42)
+    iats = []
+    for _ in range(20):
+        # Mostly fast, occasionally slow → CV in the bursty band
+        iats.append(rng.choice([0.05, 0.05, 0.05, 0.30, 0.50]))
+    out = list(extract_session(_typed_events(iats), sid="cad-bursty"))
+    obs = _of(out, "motor.keystroke_cadence")
+    assert obs.value == "bursty"
+
+
+def test_hunt_and_peck_iats_emit_hunt_and_peck() -> None:
+    # CV >= 1.5: extreme bimodal (very-fast + very-slow within burst).
+    # Most IATs are tiny; a few are ~10x the mean — drives stdev/mean above 1.5.
+    iats = [0.01] * 15 + [1.4] * 5
+    out = list(extract_session(_typed_events(iats, terminator=False), sid="cad-hp"))
+    obs = _of(out, "motor.keystroke_cadence")
+    assert obs.value == "hunt_and_peck"