Charge real freight in network.py/whatif.py; drop "ROI/day %" terminology

network.py: tools/network.py now computes freight cost from the real fuel
model (puga.shipping.round_trip_cost) for direct-jump transfers instead of
the flat 9250 AIC placeholder; gateway transfers or an explicit trip_cost
in the chain YAML still use that as a fallback. puga/network.py's combine()
docstring now flags that its "flows cancel, only freight matters" claim
only holds under a symmetric-price basis (uni30, the default) - under
--basis real it understates the benefit of physical transfer by the
bid-ask spread on the moved quantity.

whatif.py: now charges marginal freight too - only the EXTRA round trips
an addition needs beyond the base's existing traffic, so a small addition
riding along on already-necessary hauls costs ~0, while one that adds
meaningfully to shipped tonnage doesn't get a free pass.

Terminology: "profit/day ÷ capex" was being reported as "ROI/day X%",
which invites reading it as a compoundable rate. It's just 1/payback_days -
there's no reinvestment mechanism and every opportunity here is
capacity/market-limited, so it can't be annualized like a real yield.
Payback in days is now the primary number everywhere (scan.py, simulate.py,
network.py, whatif.py); see docs/decisions.md for the reasoning.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tools/simulate.py

@@ -70,7 +70,7 @@ def main():
     print(f"{plan['plan_name']}  {plan['planet_natural_id']}  COGC {plan.get('plan_cogc')}  prices: {a.cx} {a.basis}")
     print(f"Area {r['area']:.0f}/500   Profit/day {r['profit']:,.0f}   gross {r['gross']:,.0f}   degradation {r['degradation']:,.0f}   plan cost {r['plan_cost']:,.0f}   ROI {r['roi_days'] and round(r['roi_days'], 2)} d")
     nc = r["new_capex"]
-    print(f"NEW CAPEX = planned minus already built ({built or 'nothing built'}): {nc:,.0f}   ->  payback {nc / r['profit']:.2f} d = {100 * r['profit'] / nc:.1f}%/day" if r["profit"] > 0 and nc > 0 else ("NEW CAPEX: nothing to build" if nc <= 0 else "NEW CAPEX: profit <= 0"))
+    print(f"NEW CAPEX = planned minus already built ({built or 'nothing built'}): {nc:,.0f}   ->  payback {nc / r['profit']:.2f} d" if r["profit"] > 0 and nc > 0 else ("NEW CAPEX: nothing to build" if nc <= 0 else "NEW CAPEX: profit <= 0"))
     print("\nWORKFORCE   need supply  open   eff%")
     for t, w in r["workforce"].items():
         if w["need"] or w["supply"]: