fix(signal): reduce duplicate observer registrations in repeated signal reads

[oanlit]

Summary

When a signal is read repeatedly within the same computation (e.g. inside loops),
the current implementation registers the same observer multiple times.

This leads to unnecessary growth of internal tracking structures, including:

• observers
• observerSlots
• sources
• sourceSlots

This issue is especially noticeable in patterns such as:

• Tight loops:

createEffect(() => {
  for (let i = 0; i < 1000; i++) a();
});

• Iteration over store arrays:

createEffect(() => {
  for (const item of arrayStore) {
    _ = item;
  }
});

In these cases, repeated signal reads (e.g. length, indexed access, or direct signal calls)
cause the same computation to be registered multiple times.

---

This PR introduces a lightweight deduplication mechanism by tracking the last registered observer per signal.

• Adds lastObserver to SignalState
• Only registers the current computation if it differs from lastObserver
• Resets lastObserver during cleanup

This significantly reduces redundant registrations in common scenarios while keeping the cost of deduplication O(1) and preserving existing behavior.

---

How did I test this change?

To enable verification of internal observer behavior in tests, this PR slightly adjusts the return value of createSignal:

const result = [readSignal.bind(s), setter] as Signal<T | undefined>;
if (IS_DEV) (result as any).state = s;
return result;

2. Added a test case that repeatedly reads a signal inside a loop:

const a = createSignal(0);
const dispose = createRoot(dispose => {
  createEffect(() => {
    for (let i = 0; i < 1000; i++) {
      a[0]();
    }
  });
  return dispose;
});
expect((a as any).state.observers.length).toBe(1);
expect((a as any).state.observerSlots.length).toBe(1);
expect((a as any).state.observers[0].sources.length).toBe(1);
expect((a as any).state.observers[0].sourceSlots.length).toBe(1);
dispose();

Before this change:

• observers.length === 1000

After this change:

• observers.length === 1

2. Verified similar behavior with store array iteration:

const [arrayStore] = createStore([1, 2, 3, 4, 5]);

createEffect(() => {
  for (const item of arrayStore) {
    _ = item;
  }
});

Confirmed that repeated reads (e.g. length and indexed access) no longer
cause excessive observer registrations.

3. Ran full test suite:

pnpm build
pnpm test

All tests pass.

4. Manually inspected that:

• observerSlots and sourceSlots no longer grow excessively
• No regression in reactivity behavior

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