feat(profiler/behave_shell): scaffold extract_session entry point

BEHAVE-EXTRACTOR.md Phase A Step 0. Lays the package skeleton
(__init__/extract/_parse/_ctx/_thresholds/_features) with empty
FEATURES = (), so the worker plumbing in BEHAVE-INTEGRATION Phase 4
has a stable import path before any primitive lands.

extract_session() builds a SessionContext once and fans the
registered feature functions across it; at Step 0 that fan-out is
empty and the function yields nothing. Step 1 (asciinema parser +
paste-burst detector) and Step 2 (motor.input_modality) land next.

Smoke suite asserts the empty contract: empty stream → no
observations, single event → t_start == t_end, multi-event → events
routed into input_events / output_events by kind, evidence_ref
defaults to "session:<sid>" or honours an explicit override.

tests/profiler/behave_shell/test_extract_smoke.py

@@ -0,0 +1,94 @@
+"""Step 0 smoke: prove the wiring before any logic.
+
+Before any feature function lands, verify:
+
+* the public ``extract_session`` import path resolves;
+* an empty event stream yields zero observations and a well-formed
+  zero-duration ``SessionContext``;
+* a single input event yields a context with ``t_start == t_end``
+  and ``duration_s == 0.0``;
+* a multi-event stream populates ``t_start`` / ``t_end`` /
+  ``duration_s`` correctly and routes events into the
+  ``input_events`` / ``output_events`` slots by kind;
+* ``FEATURES`` is empty at Step 0 — the empty contract is the gate
+  that the next step must intentionally break.
+"""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import (
+    DEFAULT_SOURCE,
+    build_context,
+    extract_session,
+)
+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_default_source_is_canonical_path() -> None:
+    assert DEFAULT_SOURCE == "decnet/profiler/behave_shell/extract.py"
+
+
+def test_extract_session_empty_stream_yields_no_observations() -> None:
+    out = list(extract_session([], sid="sess-empty"))
+    assert out == []
+
+
+def test_build_context_empty_stream_zero_duration() -> None:
+    ctx = build_context([], sid="sess-empty")
+    assert ctx.sid == "sess-empty"
+    assert ctx.source == DEFAULT_SOURCE
+    assert ctx.evidence_ref == "session:sess-empty"
+    assert ctx.t_start == 0.0
+    assert ctx.t_end == 0.0
+    assert ctx.duration_s == 0.0
+    assert ctx.input_events == ()
+    assert ctx.output_events == ()
+
+
+def test_build_context_single_input_event() -> None:
+    events: list[AsciinemaEvent] = [(1.5, "i", "a")]
+    ctx = build_context(events, sid="sess-1")
+    assert ctx.t_start == 1.5
+    assert ctx.t_end == 1.5
+    assert ctx.duration_s == 0.0
+    assert ctx.input_events == ((1.5, "i", "a"),)
+    assert ctx.output_events == ()
+
+
+def test_build_context_multi_event_routes_by_kind() -> None:
+    events: list[AsciinemaEvent] = [
+        (0.0, "i", "l"),
+        (0.1, "i", "s"),
+        (0.2, "o", "ls\r\n"),
+        (0.3, "o", "file.txt\r\n"),
+        (0.5, "i", "\r"),
+    ]
+    ctx = build_context(events, sid="sess-multi")
+    assert ctx.t_start == 0.0
+    assert ctx.t_end == 0.5
+    assert ctx.duration_s == 0.5
+    assert len(ctx.input_events) == 3
+    assert len(ctx.output_events) == 2
+    # Order preserved
+    assert ctx.input_events[0] == (0.0, "i", "l")
+    assert ctx.output_events[-1] == (0.3, "o", "file.txt\r\n")
+
+
+def test_extract_session_explicit_evidence_ref_overrides_default() -> None:
+    ctx = build_context(
+        [(0.0, "i", "x")],
+        sid="sess-x",
+        evidence_ref="shard:/var/log/d/sess-x.cast",
+    )
+    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"))
+    assert out == []