Make count, isEmpty, first, entities and for-of honour tick filters

[srikarsunchu]

Fixes #2.

What

count, isEmpty, first, entities() and for … of on a Changed() / Added() / Removed() query now answer for the filter. The issue names the three scalar accessors; reading EntityIterator showed for … of and entities() were built from the archetype list alone and had the same gap, so all five read paths are covered together.

How — option 1 from the issue, with one property pinned down

The reads must not consume the change window, or if (!q.isEmpty) q.each(…) would cost the run its events. And a read inside a running each() must not swap the filter's bound columns from under it, which routing through the shared RowFilter would.

So each result keeps a second RowFilter over the same terms for reads. RowFilter.peek(ticks, horizon) opens it at the live filter's lastSeen without advancing anything; begin() is unchanged and now shares its body with peek(). The probe walks archetypes and rows in the same back-to-front order each() uses, so first is the entity each() would visit first.

• Unfiltered queries keep their O(1) answers; a filtered read is the scan each() already pays.
• Sorted views answer through their own ListWalk, which owns the filter their each() runs with (they used to delegate count/isEmpty to the base, whose filter is a different instance with its own lastSeen). Ordered views already delegate everything to the base, so they follow.
• One existing test (accessor.test.ts › get is not a write) asserted query.count === 1 on a Changed query as an incidental "still exists" check; it now asserts both facts explicitly (world.query(Position).count === 1, query.isEmpty).
• README queries table and SPEC §8.3 now say which operation consumes the window.

Tests

Six new cases in tick-filters.test.ts: reads answer for the filter; reading does not consume what each() is about to see (and after the run the window is closed); first follows each() order; Added/Removed; sorted views; unfiltered reads untouched.

npm test: 2348 passed across dev / prod / react / solid / types. tsc --noEmit and prettier clean.

Not in this PR

#3 (a Changed reader ordered before its writer) is a scheduling design choice between the options listed there; nothing here changes begin() semantics.

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