feat: economy with lazy stock advancement and price curve

src/sim/constants.js

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+export const GOODS = ['ore', 'food', 'fuel', 'tech', 'meds', 'luxury'];
+export const BASE_PRICE = { ore: 10, food: 8, fuel: 12, tech: 40, meds: 30, luxury: 60 };
+export const TYPICAL_STOCK = 120;   // stock at which price == base price
+export const MAX_STOCK = 2000;
+
+export const NEWS_DAYS_PER_LANE = 1;  // price news travels 1 lane per day
+export const SNAPSHOT_KEEP = 30;      // days of price history kept per planet
+
+export const DOCK_DELAY = 0.25;       // days an NPC idles between arrival and replanning
+export const INSYS_LEG_DAYS = 0.3;    // planet<->planet or planet<->gate hop inside a system
+export const OBSERVE_INTERVAL = 0.1;  // how often the player's system marks ships as seen
+export const VIS_BAND = 0.15;         // fraction of a lane near a system that is "inside" it
+
+export const WIN_CREDITS = 100000;
+export const PLAYER_START_CREDITS = 1000;
+export const PLAYER_CAPACITY = 30;

src/sim/economy.js

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+import { GOODS, BASE_PRICE, TYPICAL_STOCK, MAX_STOCK } from './constants.js';
+
+// Deterministic price curve: scarce -> expensive. Clamped to [0.25x, 4x].
+export function priceOf(good, stock) {
+  const ratio = TYPICAL_STOCK / Math.max(stock, 1);
+  const mult = Math.min(4, Math.max(0.25, ratio));
+  return Math.round(BASE_PRICE[good] * mult * 100) / 100;
+}
+
+// Lazily advance a planet's stocks to time t (no per-tick simulation).
+export function advancePlanet(planet, t) {
+  const dt = t - planet.stockTime;
+  if (dt <= 0) return;
+  for (const g of GOODS) {
+    const rate = (planet.produces[g] || 0) - (planet.consumes[g] || 0);
+    planet.stock[g] = Math.min(MAX_STOCK, Math.max(0, planet.stock[g] + rate * dt));
+  }
+  planet.stockTime = t;
+}
+
+// Execute a trade at the current price. deltaQty > 0 delivers goods,
+// deltaQty < 0 removes them. Returns the unit price the trade executed at.
+export function applyTrade(planet, t, good, deltaQty) {
+  advancePlanet(planet, t);
+  const price = priceOf(good, planet.stock[good]);
+  planet.stock[good] = Math.min(MAX_STOCK, Math.max(0, planet.stock[good] + deltaQty));
+  return price;
+}
+
+export function marketView(planet, t) {
+  advancePlanet(planet, t);
+  return GOODS.map((g) => ({
+    good: g,
+    stock: Math.floor(planet.stock[g]),
+    price: priceOf(g, planet.stock[g]),
+  }));
+}