Writes that net under a custom equals still pend (D-8) #4483

On next (ab3d2ad), writes count as netted only by identity: a signal's staging counts as netted only when _pendingValue === _value. A signal with a custom comparator can net back to a value the comparator calls equal to the committed one, and that staging still counts as a proposal. The memo over it goes pending. D-8 says writes that net to the committed value make no proposal.

const one = { k: 1 };
const [s, setS] = createSignal(one, { equals: (a, b) => a.k === b.k });
const m0 = createMemo(() => {
  const v = s().k + 1;
  if (v % 4 === 2) throw new Error(`m0:${v}`);
  return v;
});
createRenderEffect(() => isPending(m0), v => seen.push(v));
flush();
const act = action(function* () {
  setS({ k: 1 }); // equal to the committed value by the comparator, not by identity
  yield new Promise(() => {});
});
setS({ k: 2 });
act();
// await a task, then flush
// seen: [false, true]   expected: [false]

Writing back the committed object itself (setS(one)) nets, and seen stays [false].

One possible fix: at write time, when the comparator says the new staging equals the committed value, store the committed object as the staging. The existing identity test then sees the writes as netted.

Found while reviewing #4472.