Draw the + on the pixel grid so it stays sharp on 1x screens

On a 1x display TDesign's + smeared at every small size: its 24-grid
strokes are centred on the box's middle, which at 12 or 14px is a pixel
boundary, so each ~1px bar covered two pixels at half ink. A thinner
stroke alone keeps the bar on the boundary. This is not Electrobun:
headless Chrome renders the same.

pixelAdd (icons.ts) redraws the + on the 14px grid with 1px bars at 7.5,
so they fill whole pixels at 1x and 2x, half a pixel off-centre. It
replaces tdesignAdd in the tab bar, the shortcut rows, the connection
manager and the tab menu. Checked at 1x in the real tab-bar and
shortcut-row markup: single-pixel bars at full ink.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/gotchas.md

@@ -82,6 +82,14 @@ touching the adjacent code.
   it. Per-template picks had drifted to 12 sizes, and neighbours stopped
   matching: a 14.4px cog beside a 12.8px +, and a 12px + beside 9.6px × in
   the shortcut chips.
+- **On a 1x screen, a TDesign line through the icon's centre smears at any
+  small size.** At 12 or 14px the centre is a pixel boundary, so a ~1px bar
+  covers two pixels at half ink. A 1px stroke alone doesn't help, because the
+  bar stays centred on the boundary. This is geometry, not Electrobun:
+  headless Chrome renders the same. So the + is `pixelAdd` (`icons.ts`),
+  redrawn on the 14px grid with 1px bars at 7.5, which fill whole pixels at 1x
+  and at 2x. It's tuned for `--icon-size-sm` only. Arrow shafts (↑ →) have
+  the same issue if it ever shows.
 
 ## The interface scale (root font-size lever)