Extract Insta pair builder orchestration

docs/prompt-architecture-improvement-plan.md

@@ -275,13 +275,13 @@ Already isolated:
 
 ### Pair / Adapter Layer
 
-Owner today: `build_insta_of_pair`.
+Owner today: `pair_builder.py`; `prompt_builder.build_insta_of_pair` is the
+public wrapper used by the node layer.
 
 Keep here:
 
-- pair route sequencing;
-- top-level continuity option handoff between row, camera, clothing, and output
-  adapters.
+- the public wrapper signature and dependency bridge needed by existing nodes
+  and tests.
 
 Already isolated:
 
@@ -290,6 +290,10 @@ Already isolated:
   policy, plus hardcore detail-density directive text, live in
   `pair_options.py`; `prompt_builder.py` keeps public delegate wrappers for
   existing nodes and tests.
+- pair route sequencing now lives in `pair_builder.py` behind
+  `InstaPairBuildRequest` and `InstaPairBuildDependencies`, covering
+  option/filter/seed/cast parsing handoff, soft/hard row orchestration, cast
+  context, camera route, clothing route, and final output assembly delegation.
 - soft/hard row creation lives in `pair_rows.py` behind `InstaPairRowsRoute`,
   including softcore expression override resolution, Woman A slot context
   application, soft outfit/pose overrides, POV row fields, hardcore row