count, isEmpty and first ignore Changed/Added/Removed filters

[k9p5]

Summary

On a tick-filtered query, count, isEmpty and first report the structural match set and ignore the Changed() / Added() / Removed() terms. each and iteration apply the filter; the scalar accessors do not.

Repro

import { World, Trait, f32, Changed } from 'apecs';
const Position = new Trait({ x: f32(0) });
const w = new World();
w.spawn(Position); w.spawn(Position); w.spawn(Position);

const q = w.query(Changed(Position));
q.each(() => {});   // consume the initial state
w.step();

let n = 0; q.each(() => n++);
console.log(n);          // 0
console.log(q.count);    // 3
console.log(q.isEmpty);  // false
console.log(q.first);    // an entity

Cause

count, isEmpty and first in src/core/result.ts sum archetype.rows across the matched archetypes and never consult the RowFilter.

Why it matters

The README's Queries table lists query.count / .isEmpty / .first with no caveat, so world.query(Changed(Position)).isEmpty reads as a natural "anything changed?" check and silently answers the wrong question.

Options

1. Route the three accessors through the filter when one is present. count becomes a scan, which is the same cost each already pays for these queries.
2. Keep them structural, document it in the README table and spec §8, and add a dev-build assertion that throws when they are read on a tick-filtered query.

Found by a post-publish smoke test of 0.1.0.