Fix dynamic virtual scroll geometry

[rdlabo]

Summary

• replace heuristic fractional-index adjustments with exact cumulative pixel boundaries and binary searches
• make reverse scrolling read and write symmetric native coordinates for the demo's column-reverse layout
• retain the last complete viewport geometry while Angular data and size signals temporarily have different lengths
• preserve the legacy calcIndex behavior and expose calculateItemCountForPixelDistance for mathematically continuous results
• document the exact size-model contract and reverse scrolling behavior

Root cause

The strategy converted pixel distances into fractional item counts and then combined floor, ceil, and extra-item corrections. Those calculations used inconsistent coordinate origins for forward, backward, and reverse ranges, so the rendered range could only be tuned empirically.

Impact

Rendered ranges, content offsets, total content size, and scrolled indexes now derive from the same prefix-sum geometry. Normal scroll events use cached prefix sums and binary search. Reverse scrollToIndex() now converts logical positive offsets to the negative native scrollTop used by the demo's double column-reverse layout.

Validation

• npx ng test scroll-strategies --watch=false — 14 tests passed
• npm run lint — all projects passed
• npm run prebuild:scroll-strategies — production package build passed
• Chrome verification on the reverse demo: index 20 had an 886px cumulative offset, producing native scrollTop = -886 and logical scrollOffset = 886
• git diff --check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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