fix: prevent fast-scroll gaps

[ImLunaHey]

What changed

• add opt-in synchronousWheelScrolling for products that prioritize guaranteed wheel/trackpad rendering over threaded scrolling
• render cancelable wheel destinations before changing the element's scrollTop
• pre-render non-cancelable wheel destinations without double-applying native scroll
• make automatic scrollToOffset and scrollToIndex calls commit their target before moving the element
• preserve measurement-based anchor corrections during pre-scroll commits
• track the bounds of the last DOM range committed by React and synchronously recover escaped native ranges
• configure the demo with synchronous wheel input and an eight-viewport pixel buffer
• keep the demo's browser-native scrollbar
• isolate the demo viewport and rows for cheaper layout and paint
• retain a viewport-sized fallback for native touch, keyboard, and scrollbar checkerboarding
• pin development CI to the matching core virtualizer commit

Root cause

High-velocity threaded scrolling can paint a new offset before JavaScript receives a scroll event. More overscan reduces checkerboarding but cannot guarantee real content for arbitrary physical jumps.

The opt-in wheel path handles input before the browser changes the scroll position, so it can commit destination rows first. Automatic programmatic jumps use the same ordering. Native scrollbar-thumb dragging provides no cancellable event containing its target before the browser moves, so the native control is retained and that extreme-drag limitation is documented rather than replaced with a non-native scrollbar.

Validation

• full package check and frozen-lockfile install
• 25 tests with 100% statement, branch, function, and line coverage
• cancelable and non-cancelable pixel, line, and page wheel behavior covered
• measurement correction during pre-render covered
• React Doctor reports zero issues
• package audit and demo production build pass
• trusted browser wheel jumps of 100,000px and 1,000,000px remained covered
• automatic programmatic jumps commit their destination before moving the viewport
• keyed prepend anchoring remains stable