Extract Insta pair output assembly

docs/prompt-architecture-improvement-plan.md

@@ -147,14 +147,11 @@ Keep here:
 - soft/hard row creation;
 - continuity policy;
 - softcore cast policy;
-- pair metadata shape.
 
 Improve later:
 
-- make a single pair metadata sanitizer that normalizes `softcore_row`,
-  `hardcore_row`, pair prompts, negatives, captions, and camera fields;
 - split pair assembly into small functions by phase:
-  `build_soft_row`, `build_hard_row`, `assemble_pair_metadata`.
+  `build_soft_row`, `build_hard_row`.
 
 Already isolated:
 
@@ -166,6 +163,9 @@ Already isolated:
   including action-aware body-access flags, conflicting outfit-piece cleanup,
   default visible-men clothing, character-clothing override handling, and final
   root clothing-state assembly.
+- final pair output assembly lives in `pair_output.py`, including soft/hard
+  prompt strings, trigger preservation, negatives, captions, and root metadata
+  shape.
 
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