svu defaults to a 'v' tag prefix which was leaking into the Nix
derivation name (adiff-v0.5.0 instead of adiff-0.5.0). Pass
--tag.prefix "" so the unprefixed version is written to VERSION,
and correct the current VERSION file accordingly.

Add side-scrolling for lines that exceed column width. Both columns share
a single horizontal offset so alignment is preserved mid-scroll.

- keys: h/l step 8, H/L half-column jump, 0 home, $ end
- mouse: trackpad wheel left/right (step 4)
- status bar shows "col:N" when scrolled

Also fix a latent cell-width miscalculation in View(): rows were rendered
3 chars wider than the terminal (m.width/2 vs the correct (m.width-3)/2),
which caused the rightmost cell to wrap/clip. Align View's formula with
printDiff's and recompute maxHOffset the same way so $ lands exactly at
the right edge of the longest line.