fix(gui): anti-aliased rounded rects from one distance field

Corners were drawn by two disagreeing algorithms: a hand-rolled per-row inset for
the gradient fill and TFT_eSPI's Bresenham arc for the border, so they missed each
other by a pixel and left a halo. The inset was also wrong, truncating the sqrt and
ignoring pixel centres, which over-cut the top row by 2px into a visible chamfer.

gui.roundRect now derives fill and border from the same signed distance, blending
edge pixels by coverage, and replaces fillRoundRect, drawRoundRect and
fillRectGradient. The geometry moved to src/gfx/round_rect.h so round_rect_test.cpp
can check the arc on the host, since only the eye ever checked the old one.

test/ui_theme.lua

@@ -12,9 +12,7 @@ gui = {
   clear = function() end,
   fillRect = function() end,
   drawText = function() end,
-  fillRoundRect = function() end,
-  drawRoundRect = function() end,
-  fillRectGradient = function() end,
+  roundRect = function() end,
   fontHeight = function() return 8 end,
   textWidth = function(text) return #text * 6 end,
 }