feat(profiler/behave_shell): emit motor.input_modality

BEHAVE-EXTRACTOR.md Phase A Step 2. The first primitive — picked
first because it has the highest discriminative value (HUMAN vs
everyone) and the simplest implementation (paste-event ratio over
total inputs).

* _features/motor.py:input_modality(ctx) emits one Observation
  per session in {typed, pasted, mixed} with confidence 0.75 / 0.70.
* _features/_emit.py centralises the make_observation helper so
  every feature module gets the same Window/source/evidence_ref
  boilerplate without copy-paste.
* Thresholds inherited from the prototype's calibration history
  (MODALITY_PASTED_MIN=0.40, MODALITY_TYPED_MAX=0.05).
* Zero-input session skips emission — registry doesn't admit
  "unknown" here.

Tests: pure-typed → typed, pure-pasted → pasted, mixed → mixed,
output-only session → no observation, full envelope round-trip.

tests/profiler/behave_shell/test_extract_smoke.py

@@ -24,8 +24,11 @@ from decnet.profiler.behave_shell._features import FEATURES
 from decnet.profiler.behave_shell._parse import AsciinemaEvent
 
 
-def test_features_tuple_is_empty_at_step_0() -> None:
-    assert FEATURES == ()
+def test_features_tuple_is_populated() -> None:
+    # Step 2+: at least one feature is registered. Exact membership is
+    # asserted in per-feature tests; this test only pins "the registry
+    # is non-empty" so the empty-FEATURES regression doesn't sneak back.
+    assert len(FEATURES) >= 1
 
 
 def test_default_source_is_canonical_path() -> None:
@@ -87,8 +90,7 @@ def test_extract_session_explicit_evidence_ref_overrides_default() -> None:
     assert ctx.evidence_ref == "shard:/var/log/d/sess-x.cast"
 
 
-def test_extract_session_with_features_still_empty() -> None:
-    """Until Step 2 lands, even a populated stream emits nothing."""
-    events: list[AsciinemaEvent] = [(t / 10.0, "i", c) for t, c in enumerate("hello\r")]
-    out = list(extract_session(events, sid="sess-features-empty"))
+def test_extract_session_zero_inputs_yields_nothing() -> None:
+    """No input events → no feature emits (input_modality skips on empty)."""
+    out = list(extract_session([(0.0, "o", "hi\r\n")], sid="sess-no-input"))
     assert out == []

tests/profiler/behave_shell/test_motor_input_modality.py

@@ -0,0 +1,64 @@
+"""Step 2: ``motor.input_modality`` — typed / pasted / mixed."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+def _by_primitive(observations: list, primitive: str):
+    return [o for o in observations if o.primitive == primitive]
+
+
+def test_pure_typed_session_emits_typed() -> None:
+    events: list[AsciinemaEvent] = [(i * 0.1, "i", c) for i, c in enumerate("ls\r")]
+    out = list(extract_session(events, sid="sess-typed"))
+    obs = _by_primitive(out, "motor.input_modality")
+    assert len(obs) == 1
+    assert obs[0].value == "typed"
+    assert obs[0].confidence == 0.75
+
+
+def test_pure_pasted_session_emits_pasted() -> None:
+    # Three large input events, no typing
+    events: list[AsciinemaEvent] = [
+        (0.0, "i", "echo first paste\r"),
+        (1.0, "i", "echo second paste\r"),
+        (2.0, "i", "echo third paste\r"),
+    ]
+    out = list(extract_session(events, sid="sess-pasted"))
+    obs = _by_primitive(out, "motor.input_modality")
+    assert len(obs) == 1
+    assert obs[0].value == "pasted"
+
+
+def test_mixed_session_emits_mixed() -> None:
+    # 1 paste event + 9 single-char typed events → ratio 0.10 → in
+    # between the typed (≤0.05) and pasted (≥0.40) thresholds → mixed
+    events: list[AsciinemaEvent] = [(0.0, "i", "echo hello\r")]
+    events += [(0.5 + i * 0.1, "i", c) for i, c in enumerate("ls -la\rps\r")]
+    out = list(extract_session(events, sid="sess-mixed"))
+    obs = _by_primitive(out, "motor.input_modality")
+    assert len(obs) == 1
+    assert obs[0].value == "mixed"
+
+
+def test_zero_input_session_emits_nothing_for_modality() -> None:
+    # Output-only session: no honest answer, so we don't emit.
+    events: list[AsciinemaEvent] = [(0.0, "o", "welcome\r\n")]
+    out = list(extract_session(events, sid="sess-empty-input"))
+    assert _by_primitive(out, "motor.input_modality") == []
+
+
+def test_observation_envelope_fields_are_populated() -> None:
+    events: list[AsciinemaEvent] = [(0.0, "i", "echo paste paste\r")]
+    out = list(extract_session(
+        events, sid="sess-env", evidence_ref="shard:/blob/sess-env",
+    ))
+    obs = _by_primitive(out, "motor.input_modality")[0]
+    assert obs.source == "decnet/profiler/behave_shell/extract.py"
+    assert obs.evidence_ref == "shard:/blob/sess-env"
+    assert obs.window.start_ts == 0.0
+    assert obs.window.end_ts == 0.0
+    # envelope auto-populates id / ts / v
+    assert obs.id and len(obs.id) > 0
+    assert obs.v == 1