feat: balance checks and README - v1 complete

README.md

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+# Probability
+
+A neon space-trading sim where unobserved ships are probability waves.
+
+Inspired by Space Rangers. Every NPC trader plans real routes and real trades,
+but between observations its position is a probability cloud smeared along the
+jump lanes — and so is your knowledge of every market more than a news-day away.
+Observation collapses the cloud: enter a system and ships condense out of the
+static; dock at a planet and its prices become real.
+
+Under the hood the universe is deterministic ("hidden variables" drawn lazily
+from per-ship seeds) — the probability is *yours*, not the universe's.
+
+## Run
+
+    npm install
+    npm run dev        # play at http://localhost:5173 (add ?seed=anything for a new galaxy)
+    npm test           # headless sim test suite
+
+## How to play
+
+- You start docked. Buy what a planet produces cheap, sell where it's consumed.
+- Sidebar: navigation, market (when docked), cargo, intel, log. Space = pause,
+  Tab = map/system view. Click planets/gates/systems to move; click systems on
+  the map for price intel — note the age tag, old news lies.
+- Lanes are tinted by pirate danger. Dangerous shortcuts are profitable until
+  they aren't.
+- Orange smears on the map are ships you've met: where they probably are now.
+- Reach 100,000 credits to win. Lose your ship and it's over.

tests/balance.test.js

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+import { describe, it, expect } from 'vitest';
+import { createWorld, advance } from '../src/sim/world.js';
+import { priceOf, advancePlanet } from '../src/sim/economy.js';
+import { GOODS, BASE_PRICE } from '../src/sim/constants.js';
+
+describe('balance', () => {
+  it('the NPC economy stays alive over a long horizon', () => {
+    const w = createWorld('balance-1');
+    advance(w, 120);
+    const alive = w.npcs.filter((n) => n.alive);
+    expect(alive.length).toBeGreaterThan(w.npcs.length * 0.5);
+    expect(w.stats.trades).toBeGreaterThan(w.npcs.length * 3);
+  });
+
+  it('price gradients survive NPC arbitrage (the player can still profit)', () => {
+    const w = createWorld('balance-1');
+    advance(w, 60);
+    let bestSpread = 0;
+    for (const good of GOODS) {
+      let lo = Infinity, hi = 0;
+      for (const planet of w.galaxy.planets) {
+        advancePlanet(planet, w.t);
+        const price = priceOf(good, planet.stock[good]);
+        lo = Math.min(lo, price);
+        hi = Math.max(hi, price);
+      }
+      bestSpread = Math.max(bestSpread, (hi - lo) / BASE_PRICE[good]);
+    }
+    expect(bestSpread).toBeGreaterThan(0.5); // at least one good still has a fat margin somewhere
+  });
+});

vite.config.js

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+import { defineConfig } from 'vite';
+
+export default defineConfig({
+  build: { target: 'esnext' }, // top-level await in src/main.js
+});