refactor(ui): restore native scrollbind

[esmuellert]

Summary

• Remove the custom structural scroll-sync engine and its per-tab manager.
• Restore Neovim's native scrollbind/syncbind setup for side-by-side and conflict views.
• Keep refresh, lifecycle, and moved-block alignment behavior working with native scrollbind.
• Clear inherited scrollbind from Explorer and user-created split windows, and use session-owned windows during refresh.
• Replace engine-specific specs with native scrollbind coverage, including the tall virt_lines regression case.

Why

Neovim's tall-virt_lines scrollbind regression was fixed upstream in Neovim PR #41519. The structural scroll-sync engine also introduced a second scroll path that conflicts with Snacks' smooth-scroll animation, which is the cause of #519. CodeDiff no longer needs that parallel path, so this PR restores the native mechanism and removes the conflicting WinScrolled event, virtual-row mapping, and echo-management code.

Fixes #519

Testing

• cmake -B build -S . && cmake --build build --parallel 5
• ./tests/run_tests.sh (109 spec files, all passed)
• Headless scroll stress test on Neovim 0.13-dev: 500 downward and 500 upward scroll steps across large insertion, deletion, and leading-insertion diffs; maximum backward jump was 0.
• git diff --check