Keep the clicked row visible when the diff pane opens over it

[danipen]

Problem

Clicking a branch label (or commit) near the bottom of the Graph opens the diff pane under the stage — and the shrunk canvas swallowed the very thing that was just clicked. The selection ended up hidden behind the pane.

Fix

Arm a one-shot reveal when the selection changes: the resize the opening pane triggers consumes it and pans vertically, just enough to keep the clicked row — label pill, nodes and caption — above the fold.

• Never a recenter or horizontal jump, so the user keeps their bearings; a row already clear of the pane doesn't move at all.
• The reveal expires two frames after the selection, so splitter drags and window resizes stay entirely user-driven (they can still cover the selection — by the user's own hand, with no surprise pan).
• The pan math lives in a pure revealRowDy(view, viewportHeight, row) in geometry.ts (unit-tested); GraphCanvas.tsx wires it into the existing ResizeObserver.

Validation

• lint, typecheck, full bun test suite green (6 new tests for revealRowDy).
• Drove the real app over CDP and reproduced the original scenario: clicking a bottom branch label now leaves the branch fully visible right above the pane, for both branch-label and commit-node clicks, with no console errors.

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