feat(profiler/behave_shell): emit temporal.session_duration

Bucket ctx.duration_s against SESSION_DURATION_SHORT_MAX (60s) /
MEDIUM_MAX (600s) / LONG_MAX (3600s); else marathon. Direct
measurement, confidence 0.85. Skip emission only when no commands
and zero duration. New _features/temporal.py module opens Phase E.

tests/profiler/behave_shell/test_temporal_session_duration.py

@@ -0,0 +1,50 @@
+"""Step E.1: ``temporal.session_duration``."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+PRIMITIVE = "temporal.session_duration"
+
+
+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 test_empty_session_no_emission() -> None:
+    out = list(extract_session([], sid="dur-empty"))
+    assert [o for o in out if o.primitive == PRIMITIVE] == []
+
+
+def test_under_60s_emits_short() -> None:
+    events: list[AsciinemaEvent] = [(0.0, "i", "a"), (30.0, "i", "b")]
+    obs = _of(list(extract_session(events, sid="dur-short")), PRIMITIVE)
+    assert obs.value == "short"
+
+
+def test_under_600s_emits_medium() -> None:
+    events: list[AsciinemaEvent] = [(0.0, "i", "a"), (300.0, "i", "b")]
+    obs = _of(list(extract_session(events, sid="dur-med")), PRIMITIVE)
+    assert obs.value == "medium"
+
+
+def test_under_3600s_emits_long() -> None:
+    events: list[AsciinemaEvent] = [(0.0, "i", "a"), (1800.0, "i", "b")]
+    obs = _of(list(extract_session(events, sid="dur-long")), PRIMITIVE)
+    assert obs.value == "long"
+
+
+def test_over_3600s_emits_marathon() -> None:
+    events: list[AsciinemaEvent] = [(0.0, "i", "a"), (7200.0, "i", "b")]
+    obs = _of(list(extract_session(events, sid="dur-marathon")), PRIMITIVE)
+    assert obs.value == "marathon"
+
+
+def test_high_confidence() -> None:
+    """Duration is a fact, not an inference — confidence stays high."""
+    events: list[AsciinemaEvent] = [(0.0, "i", "a"), (30.0, "i", "b")]
+    obs = _of(list(extract_session(events, sid="dur-conf")), PRIMITIVE)
+    assert obs.confidence >= 0.80