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.

decnet/profiler/behave_shell/extract.py

@@ -0,0 +1,51 @@
+"""Public extraction entry point.
+
+``extract_session`` is the only function workers call. It builds a
+:class:`SessionContext` once and fans the registered feature functions
+across it. Pure library: no I/O, no bus, no DB. The worker
+(``BEHAVE-INTEGRATION.md`` Phase 4) is responsible for those.
+"""
+from __future__ import annotations
+
+from typing import Iterable, Iterator
+
+from decnet_behave_core.spec.envelope import Observation
+
+from decnet.profiler.behave_shell._ctx import SessionContext, build_session_context
+from decnet.profiler.behave_shell._features import FEATURES
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+DEFAULT_SOURCE = "decnet/profiler/behave_shell/extract.py"
+
+
+def extract_session(
+    events: Iterable[AsciinemaEvent],
+    *,
+    sid: str,
+    source: str = DEFAULT_SOURCE,
+    evidence_ref: str | None = None,
+) -> Iterator[Observation]:
+    """Yield BEHAVE-SHELL observations for a single session.
+
+    ``events`` is an iterable of ``(t, kind, data)`` tuples — see
+    ``_parse.AsciinemaEvent``. ``sid`` identifies the session for
+    evidence pointers and downstream joins.
+    """
+    ctx = build_session_context(
+        events, sid=sid, source=source, evidence_ref=evidence_ref
+    )
+    for feature_fn in FEATURES:
+        yield from feature_fn(ctx)
+
+
+def build_context(
+    events: Iterable[AsciinemaEvent],
+    *,
+    sid: str,
+    source: str = DEFAULT_SOURCE,
+    evidence_ref: str | None = None,
+) -> SessionContext:
+    """Expose the SessionContext build for tests + future debug tools."""
+    return build_session_context(
+        events, sid=sid, source=source, evidence_ref=evidence_ref
+    )