perf(ui): reduce split/sidebar layout thrash during drag settle (Fluid F4)

[ZhuchkaTriplesix]

Parent

Part of #356 · Fluid perf audit F4 (P1)

Problem

Sidebar width and vertical SQL/results split update constraints/flex every drag and settle tick. Paint isolation via RepaintBoundary on main split is good, but layout of editor + results still runs every frame during fling settle.

Where:

• lib/features/main_screen/main_screen.dart
• lib/core/layout/vertical_split_pane.dart
• lib/core/layout/querya_drag_settle.dart

Expected

• Keep 1:1 mid-drag (do not animate mid-drag)
• Mitigate settle cost where cheap (e.g. RepaintBoundary on vertical panes; optional notify throttle that does not break feel)
• Document if remaining relayout is accepted

Acceptance

• Vertical split panes paint-isolated if missing
• No mid-drag spring regression
• Note in PR what was measured / accepted

Related

• fix(ui): persist sidebar width on keyboard resize and flush settle; dropdown exit animation #354 sidebar persist (shipped)
• Checklist: docs/perf-baseline.md §15 Split settle

[ZhuchkaTriplesix]

Fixed and merged on dev via Fluid perf PRs.