feat(profiler/behave_shell): G.7 emotional_valence.stress_response

Compare median post-error intra-command IATs against baseline
(commands not immediately following an errored command):

* ratio ≥ STRESS_EUSTRESS_RATIO_MIN (1.20) → eustress_positive
* ratio ≤ 1/STRESS_DISTRESS_RATIO_MIN     → distress_negative
* otherwise                                → none

Confidence hard-capped at 0.5; 0.30 below
STRESS_MIN_ERRORED_WITH_IATS (2).

decnet/profiler/behave_shell/_features/__init__.py

@@ -26,6 +26,7 @@ from decnet.profiler.behave_shell._features.cognitive import (
 )
 from decnet.profiler.behave_shell._features.emotional_valence import (
     arousal,
+    stress_response,
     valence,
 )
 from decnet.profiler.behave_shell._features.environmental import (
@@ -97,4 +98,5 @@ FEATURES: tuple[FeatureFn, ...] = (
     multi_actor_indicators,
     valence,
     arousal,
+    stress_response,
 )