Shrink warship shell sprite to a single pixel

The shell (unit-atlas.png col 7) was a centered 3×3 white square, so it
rendered as a 3×3-world-tile block. Replace it with a single centered
white pixel so shells render as one pixel, matching the original
pixel-shell look. The atlas is sampled with NEAREST and unitSize is 13
(1 atlas px ≈ 1 world tile), so the lone pixel stays crisp.

Update the UnitPass header comments that described the shell as 3×3.

src/client/render/gl/passes/UnitPass.ts

@@ -4,9 +4,9 @@
  * Renders all mobile (non-structure) units: boats, nukes, shells, SAM
  * missiles, and MIRV warheads. All unit types are rotationally symmetric
  * — no rotation needed. Sprites are tiny grayscale PNGs colorized on the
- * GPU using the standard 3-band gray replacement (180/130/70). Shell and
- * MIRV Warhead use programmatic 3×3 white squares (colorized to border
- * color).
+ * GPU using the standard 3-band gray replacement (180/130/70). MIRV
+ * Warhead uses a programmatic 3×3 white square (colorized to border
+ * color); Shell is a single white pixel.
  *
  * Two instanced draw calls per frame — ground units and missiles are
  * split into separate buffers for correct layer ordering:
@@ -21,7 +21,7 @@
  *   Col 4: Hydrogen Bomb (9×9)
  *   Col 5: MIRV (13×13, grayscale colorized)
  *   Col 6: SAM Missile (3×3)
- *   Col 7: Shell (3×3 white square)
+ *   Col 7: Shell (1×1 white pixel)
  *   Col 8: MIRV Warhead (3×3 white square)
  *   Col 9: Train Engine (5×5)
  *   Col 10: Train Carriage (5×5)