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 == []