add seeded prng: Math.seedRandom() and lib/random.ts lcg

[cs01]

Summary

• Add Math.seedRandom(seed) to codegen — converts double seed to i64 and calls srand48, making subsequent Math.random() calls deterministic for a given seed
• Add lib/random.ts — standalone Random class implementing the Numerical Recipes LCG (state = state * 1664525 + 1013904223 mod 2^32), safe within double precision

API

// Seed the global Math.random()
Math.seedRandom(42);
const r = Math.random(); // deterministic

// Standalone seeded RNG
import { Random } from "../lib/random.js";
const rng = new Random(12345);
rng.next();           // [0, 1)
rng.nextInt(0, 10);   // integer in [0, 10)

Test plan

• math seed random — same seed produces same sequence, different seed produces different value
• random lib basic — reproducibility, range checks, nextInt bounds