perf: replace JSON stringify/parse deep equality with recursive comparison

[natsukingly]

Summary

deepEqual in @x402/core used JSON.stringify → JSON.parse → JSON.stringify for comparison, creating unnecessary intermediate objects. This PR replaces it with a direct recursive comparison.

Changes

• Rewrote deepEqual to use recursive structural comparison
• Eliminates double JSON serialization/deserialization overhead
• Handles all existing edge cases: primitives, null, undefined, arrays, nested objects, key-order independence

Before

const normalize = (obj) => {
  // JSON.stringify → JSON.parse → JSON.stringify for each nested value
};
return normalize(obj1) === normalize(obj2);

After

// Direct recursive comparison — no serialization
if (Array.isArray(obj1)) return obj1.every((item, i) => deepEqual(item, obj2[i]));
return keys1.every(key => deepEqual(obj1[key], obj2[key]));

Test plan

• Existing utils.test.ts tests pass
• Full monorepo build passes