Panic on huge -width: makeslice len out of range in side-by-side rendering

[peczenyj]

Symptom

structalign -diff=side -width=4611686018427387904 ./_example
panic: runtime error: makeslice: len out of range

A user-supplied -width propagates unclamped into internal/ui:

• truncPad → runewidth.FillRight(s, w) tries to allocate w bytes
• renderSideBySide's divider → strings.Repeat("─", p.Width) likewise

Origin

Found by the ClusterFuzzLite integration work (#99): fuzz_trunc_pad crashed within 5 seconds in the OSS-Fuzz container. The bug is pre-existing — the pre-#94 hand-rolled loop had the identical strings.Repeat(" ", w-total) panic.

Fix

Clamp the column width to a sane maximum (far wider than any terminal) in truncPad (totality of the fuzzed contract) and in renderSideBySide (covers the divider). Regression test included.