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/progression_design.md

@@ -373,13 +373,13 @@ in `requirements.md` and the git history). Still open:
    one matching deposit tile). Touches REQ-BLD-MINER ("every asteroid
    tile is equivalent" no longer holds), REQ-GW-ASTEROID-EXPAND /
    REQ-EXP-*, `world.toml`, and `visuals.toml`.
-6. **Exclude scrap-consuming recipes from item threat.** Amend
-   REQ-THREAT-ITEM: recipes that take scrap as an input do not
-   participate in an item's threat computation (neither as its value
-   nor in the max-across-recipes rule). Otherwise the scrap→ingot
-   smelter recipe inflates the basic materials' threat via the max
-   rule, poisoning every downstream value. Scrap conversion is already
-   accounted for by the REQ-THREAT-SCRAP constant.
+6. **Scrap-consuming recipes as threat fallback only.** Amend
+   REQ-THREAT-ITEM: recipes that take scrap as an input participate in
+   an item's threat computation only if no scrap-free recipe (miner,
+   smelter, or assembler) produces that item — mirroring the existing
+   rule for the reprocessing path. Otherwise the scrap→ingot smelter
+   recipe would inflate the basic materials' threat via the
+   max-across-recipes rule, poisoning every downstream value.
 7. **Threat report tool.** Port the numbers-pass calculator to
    `tools/threat_report.py`: per-item threat values, module
    contributions, fitted ship threats vs. the ladder targets,