Fix ThreatCostCalculator: per-unit division, scrap fallback, fixpoint, staggered-recipe max

docs/progression_design.md

@@ -373,31 +373,3 @@ 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. **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. **Per-unit item threat.** Amend REQ-THREAT-ITEM and
-   `ThreatCostCalculator`: a recipe's threat is divided by its output
-   amount, so item threat is production-seconds *per unit*. Currently a
-   recipe producing 2 copper_wire per run assigns each wire the full
-   run's threat, double-pricing multi-output items and everything
-   downstream of them.
-8. **Fixpoint resolution in ThreatCostCalculator.** Items downstream of
-   reprocessing-only items (e.g. capital parts built from the scrap-only
-   input) never resolve, because resolution stops after the reprocessing
-   pass instead of iterating; their consumers silently drop the missing
-   materials, so capital hull threat is currently underestimated (found
-   by `tools/threat_report.py`, which implements the correct fixpoint).
-9. **Max rule across staggered recipes in ThreatCostCalculator.** An
-   item is committed at the first iteration where *any* of its recipes
-   resolves, taking the max only over the recipes resolvable at that
-   point. A shallow shortcut recipe (e.g. steel plate from raw ore)
-   resolves one iteration earlier than the base path and wins, silently
-   underpricing the item and everything downstream — violating the
-   "shortcuts are pure rewards" rule. Fix: commit an item's threat only
-   once every eligible recipe for it is computable (as
-   `tools/threat_report.py` does), with a fallback for recipe cycles.