Fix scroll capture pixel rect sampling

[yvette-carlisle]

Summary

• freeze one authoritative pixel rect when scroll capture starts
• sample live scroll frames through the frozen pixel rect instead of re-rounding the selection rect
• keep fallback sampling on the same pixel dimensions and bump the host FFI ABI

Verification

• cargo make fmt-check
• cargo make check-rust
• cargo make check-swift
• cargo make check-vstyle
• cargo make test-host-reset
• cargo make test-rust
• local app run from the branch; user verified GitHub long scroll capture no longer fails to stitch