Extract Insta pair cast context

docs/prompt-architecture-improvement-plan.md

@@ -144,18 +144,18 @@ Owner today: `build_insta_of_pair`.
 
 Keep here:
 
-- continuity policy;
-- softcore cast policy;
-
-Improve later:
-
-- split remaining pair cast/descriptor policy out of `build_insta_of_pair`.
+- pair route sequencing;
+- top-level continuity option handoff between row, camera, clothing, and output
+  adapters.
 
 Already isolated:
 
 - soft/hard row creation lives in `pair_rows.py`, including softcore expression
   override resolution, Woman A slot context application, soft outfit/pose
   overrides, POV row fields, and hardcore row creation.
+- pair-level cast/display context lives in `pair_cast.py`, including shared
+  descriptors, same-cast softcore descriptor text, partner styling, platform and
+  level labels, softcore cast presence text, and hard cast summary text.
 - pair-level camera routing lives in `pair_camera.py`, including soft/hard
   camera config selection, same-as-softcore mode, camera-detail override,
   same-room hard scene continuity, camera-aware composition mutation, POV camera