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/_features/__init__.py

@@ -0,0 +1,20 @@
+"""Registered feature functions.
+
+Each entry takes a ``SessionContext`` and yields zero or more
+``Observation`` instances. Adding a primitive = adding a function in a
+sibling module and appending it to ``FEATURES``.
+
+Step 0 ships an empty tuple — extract_session() is wired but emits
+nothing until Step 2.
+"""
+from __future__ import annotations
+
+from typing import Callable, Iterable
+
+from decnet_behave_core.spec.envelope import Observation
+
+from decnet.profiler.behave_shell._ctx import SessionContext
+
+FeatureFn = Callable[[SessionContext], Iterable[Observation]]
+
+FEATURES: tuple[FeatureFn, ...] = ()