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/_thresholds.py

@@ -0,0 +1,11 @@
+"""Numeric thresholds for BEHAVE-SHELL primitive classification.
+
+Each constant added here cites its calibration source. When the
+registry's ``notes:`` field disagrees with a constant in this file the
+registry is authoritative — fix the constant and re-run the
+calibration grid.
+
+Step 0 ships this file empty by design; thresholds land alongside the
+feature functions that consume them (Steps 1+).
+"""
+from __future__ import annotations