Fix ThreatCostCalculator: per-unit division, scrap fallback, fixpoint, staggered-recipe max (action items 6-9)

Four algorithm fixes to bring ThreatCostCalculator.cpp into agreement with
tools/threat_report.py and the newly amended REQ-THREAT-ITEM semantics:

6. Scrap-consuming recipes as threat fallback only. Non-reprocessing recipes
   that take scrap as an input are excluded from an item's threat computation
   whenever at least one scrap-free recipe (miner/smelter/assembler) produces
   that item. Previously the scrap_smelting recipe (1 scrap → 1 iron_ingot)
   would have inflated iron_ingot's threat via the max rule.

7. Per-unit item threat. computeRecipeThreatPerUnit() now divides by the
   recipe's output amount, so multi-output recipes price each unit correctly.
   Example: copper_wire (1 copper_ingot, 1 s, output 2) is now 1.5, not 3.

8. Fixpoint resolution. The resolution loop now alternates the non-reprocessing
   pass and the reprocessing pass until neither makes progress, rather than
   running the reprocessing pass once at the end. Items downstream of
   reprocessing-only items (voidsteel_plate, capital_core, capital hulls,
   drone_hangar_module) now resolve correctly.

9. Max rule across staggered recipes. An item is committed only once every
   eligible recipe producing it is computable, so a shallow shortcut recipe
   (e.g. shortcut_steel_plate: 3 iron_ore → 1 steel_plate, resolvable one
   iteration earlier) cannot undercut the expensive base path. A deadlock
   fallback (require_all_recipes=False) handles potential recipe cycles.

docs/requirements.md: REQ-THREAT-ITEM amended for per-unit division, the
scrap-fallback rule, and order-independence via fixpoint.

docs/progression_design.md: action items 6-9 removed (completed); remaining
items 1-5 renumbered unchanged.

tools/threat_report.py: NOTE updated — C++ now matches Python semantics.

bin/test/data/config/recipes.toml: four minimal test recipes added (one per
fix: scrap_iron, dual_wire, downstream_product, staggered_item_{cheap,expensive}).

src/test/ThreatCostCalculatorTest.cpp: four new TEST_CASEs covering each fix.

Expected values with the live config (bin/app/data/config) verified by
threat_report.py: iron_ingot 2, copper_wire 1.5, steel_plate 7, control_chip
12, voidsteel_plate 141, capital_core 240; fitted ships 10.5/47/99/233.5/
354.5/722.5/1491.5/1436.5. All 378 test cases pass.

tools/threat_report.py

@@ -23,10 +23,11 @@ Reads recipes.toml, ships.toml, modules.toml, and world.toml and prints:
   5. Belt feasibility — input demand in items/s per building vs. the
      single-belt cap (belt_speed_mps / tile_size_m, in items/s).
 
-NOTE: the per-unit division (1) and the scrap fallback rule are the
-agreed design semantics (see docs/progression_design.md action items);
-src/lib/sim/ThreatCostCalculator.cpp does not implement them yet. Until
-it does, this report is the design reference, not a mirror of the game.
+NOTE: the semantics described above — per-unit division, scrap fallback
+rule, fixpoint resolution including reprocessing, and the commit-all-
+recipes max rule — are implemented in both this script and in
+src/lib/sim/ThreatCostCalculator.cpp. The two should produce identical
+values for any given config.
 
 Usage (from the repository root or anywhere else):