Show the scrollbar of result formulas wider than the screen

[IACBI]

Problem

On a phone, the L and U of a 5×5 LU result (and wide result checks such as L·U = A with large entries) were cut at the right edge. They could be scrolled by swiping or Shift+wheel, but nothing showed that more was there: Flutter draws no scrollbar for a horizontal scroll view, and a matrix cut at the edge looks complete. Found in the live-site check at 390 px.

Change

• SidewaysFormula (in solution_summary.dart) wraps each result part and each result check: a horizontal scroll view with an always-visible scrollbar, like the matrix grid's.
• Room for the thumb (10 px) is reserved only while the formula overflows, decided from the scroll metrics after layout, so formulas that fit keep their spacing and show no thumb.
• With a mouse, dragging the thumb is also a way to reach the hidden part (content itself is not mouse-draggable).
• docs/DESIGN_SYSTEM.md records the rule.

Verification

• test/result_scrollbar_test.dart (4 tests): a phone-width LU result overflows and a mouse drag on the thumb scrolls it; a wide check has the always-visible bar driving its own position; a fitting result keeps zero padding; the padding appears on a phone and goes on a wide window. All four fail without the change.
• flutter analyze: no issues. flutter test: 501 passed. dart format check over all tracked files: no changes.
• tool/all_shapes_sweep_test.dart: 63/63 passed.
• Local release web build (--base-href /matriks/) in Chrome: at 390 px the bar shows under L and U in light and dark, dragging it reveals L's fifth column; at 1440 px a 2×2 LU shows no thumb and unchanged spacing; no console errors.

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