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.

decnet/profiler/behave_shell/_features/motor.py

@@ -0,0 +1,45 @@
+"""``motor.*`` feature functions.
+
+Step 2: ``motor.input_modality`` — typed / pasted / mixed.
+Step 3: ``motor.paste_burst_rate`` — none / occasional / habitual.
+"""
+from __future__ import annotations
+
+from typing import Iterator
+
+from decnet_behave_core.spec.envelope import Observation
+
+from decnet.profiler.behave_shell._ctx import SessionContext
+from decnet.profiler.behave_shell._features._emit import make_observation
+from decnet.profiler.behave_shell._thresholds import (
+    MODALITY_PASTED_MIN,
+    MODALITY_TYPED_MAX,
+)
+
+
+def input_modality(ctx: SessionContext) -> Iterator[Observation]:
+    """Emit ``motor.input_modality`` ∈ {typed, pasted, mixed}.
+
+    Ratio of paste-class events to total inputs. Empty input → skip
+    emission entirely (the registry doesn't admit ``unknown`` here
+    and fabricating ``typed`` for a zero-input session is dishonest).
+    """
+    n = len(ctx.input_events)
+    if n == 0:
+        return
+    ratio = ctx.paste_event_count / n
+    if ratio >= MODALITY_PASTED_MIN:
+        modality = "pasted"
+        confidence = 0.75
+    elif ratio <= MODALITY_TYPED_MAX:
+        modality = "typed"
+        confidence = 0.75
+    else:
+        modality = "mixed"
+        confidence = 0.70
+    yield make_observation(
+        ctx,
+        primitive="motor.input_modality",
+        value=modality,
+        confidence=confidence,
+    )