f' = 0.25 + f/2 on every band (x1.0/1.8/3.2/5.0/7.0): with the origin buy
fixed at x0.5, per-unit profit lands at exactly ~half the original in every
band (30->17, 67->35, 130->66, 218->110, 382->192 cr for the average
product) while distance stays the dominant term and the curve's shape is
preserved. Data-only change (economy.json); docs/ECONOMY.md table + example
updated. All invariants re-verified (anti-loop, monotone distance, far pays
more than buy, 1-hop viable) and the economy/cargo/shop-UI/scout suites
pass. dev/economics-report.mjs gains an ECON_BANDS=... A/B override and
dev/economy-bands-audit.mjs sweeps the band invariants on any band set.
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