fix: prevent scrolling rows from bleeding into frozen trailing band

[annahaa874]

Problem

When a cell in a freezeTrailingRows row uses span, two visual bugs appear:

1. Content bleed on horizontal scroll. walkRowsInCol draws scrolling rows all the way to the canvas bottom — past the frozen boundary. The frozen row normally paints over that overflow, but a spanning cell only fills once (first column), so subsequent columns never overwrite the bleed.

2. Partial highlight/selection. Same root cause — per-column accent (selection, highlight) is skipped by the span short-circuit, so only the first non-sticky column highlights.

Repro

• freezeTrailingRows: 1 (or higher)
• getCellContent returns a cell with span: [a, b] for the frozen row
• Enough scrolling rows so the last one crosses the freeze boundary
• Scroll horizontally → scrolling-row data leaks inside the span
• Select the row → only the first column of the span highlights

Fix

Three changes that prevent scrolling rows from entering the frozen band at all:

1. Clamp walkRowsInCol at freezeY. The scrolling-row walk loop now stops at height - freezeTrailingRowsHeight instead of height. Scrolling rows never enter the frozen zone.

2. Clip straddling rows. If a scrolling row's rendered rectangle still crosses freezeY (sub-pixel rounding), ctx.clip it at the boundary during full redraws.

3. Exempt sticky spans from the clip. Sticky columns are re-composited every frame. Clipping them would accumulate a visible seam. They don't need clipping because frozen trailing rows paint over them during the sticky pass.

Test

Regression test in test/data-editor.test.tsx: renders a frozen trailing row with a spanning cell and asserts fillRect is called more than once at the frozen row's y. Fails on main; passes with the fix. Full suite passes.

Co-authored-by: Sassoun Derderian sassound2@gmail.com