`loose.includes(entry)` ran once per LETTER inside the band loop — O(L²)
on a dense band of hundreds of loose letters, recomputed on every layer
re-render. splitYearLetters now also returns its `byEvent` map, so a
letter's disposition is `byEvent.has(linkedEventId)` and an event's card
is `byEvent.get(eventId)`, both O(1); `consumed` is a plain object. No
behavior change. Fixes review finding #3.
Refs #850
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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