Vue watch() — Reactive Watcher

watch() is Vue's API for reacting to reactive state changes with access to both the new and old values.

const state = reactive({ count: 0 });

watch(
  () => state.count,       // source — a getter function
  (newVal, oldVal) => {    // callback — called on each change
    console.log(`${oldVal} → ${newVal}`);
  }
);

state.count = 5;  // logs "0 → 5"
state.count = 10; // logs "5 → 10"

Key behaviour:

• The callback is NOT called immediately — only when the source changes.
• The callback receives (newValue, oldValue).
• The source is tracked automatically — when a reactive property accessed inside the source getter changes, the watcher re-runs.

Your task

Implement watch(source, callback), reactive(obj), and _reset().

The solve(op, ...args) harness is provided — do not modify it.