fix: replace IntersectionObserver with scroll-position check for infinite scroll

The IntersectionObserver approach for infinite scroll was unreliable —
items would not load when scrolling to the bottom in v3, while v1's
polling approach worked fine. The issue was that IntersectionObserver
with a custom root element (main-content, whose height comes from flex
align-items:stretch rather than an explicit height) didn't fire
reliably, and renderItems() being called 3 times per fetch cycle
(from both items-updated and loading-state-changed events) kept
destroying and recreating the observer.

Replace with a simple scroll-position check in the existing onscroll
handler, matching v1's proven approach: when the user scrolls within
200px of the bottom of #main-content, trigger loadMore(). This runs
on every scroll event (cheap arithmetic comparison) and only fires
when content actually overflows the container.

Remove the unused itemObserver module-level variable.

Update regression tests to simulate scroll position instead of
IntersectionObserver callbacks, with 4 cases: scroll near bottom
triggers load, scroll far from bottom doesn't, loading=true blocks,
and hasMore=false hides sentinel.

https://claude.ai/code/session_01DpWhB9uGGMBnzqS28HxnuV

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