Scrap-consuming recipes as threat fallback, not excluded

Refine the REQ-THREAT-ITEM fix per review: recipes taking scrap as
input participate in an item threat computation only when no
scrap-free recipe produces that item, mirroring the existing
reprocessing-path rule instead of a blanket exclusion.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DyCu8vwChKMbLJQ3xosYEN

docs/content_design.md

@@ -430,9 +430,10 @@ typical loadouts double as the `default_modules` for enemy waves):
 - *Rule bug discovered:* the scrap→iron_ingot smelter recipe combined
   with REQ-THREAT-ITEM's max-across-recipes rule would set
   threat(iron_ingot) to the scrap path (1 + 4 = 5, or more at other
-  scrap values) instead of 2, inflating every downstream item. Fix
-  required: exclude scrap-consuming recipes from item threat
-  computation (see progression_design.md action items).
+  scrap values) instead of 2, inflating every downstream item. Fix:
+  scrap-consuming recipes count toward an item's threat only when no
+  scrap-free recipe produces that item, mirroring the reprocessing-path
+  rule (see progression_design.md action items).
 
 ## Balancing targets (first pass, July 2026)