Add code components 366-385; ScrollArea Follow, editor sync

A terminal with history, a Go-colored code block, a log viewer that
follows, a hex viewer, a memory map, a JSON tree, a line diff, a GAME
OVER crash screen, an FPS meter, profiler bars, breakpoints,
variables, command history, a shortcut list, a debug grid, a pixel
editor that exports string art, a palette editor, a sprite sheet, a
glyph map and a 6502 register view.

layout.ScrollArea takes Follow, and ChatLog now uses it. data.TreeNode
takes a Value drawn in its Slot. pixel gains Has, and Sprite.Art
writes MustSprite art. pixel.Editor takes the caller's value at each
input event, so a value changed between frames is never typed onto.

pixel/art_internal_test.go

@@ -0,0 +1,21 @@
+package pixel
+
+import "testing"
+
+func TestArtRoundTrips(t *testing.T) {
+	art := "#a.\n.kr\n\n...\nmA#\n"
+	sp := MustSprite(art)
+	if got := sp.Art(); got != art {
+		t.Fatalf("Art() = %q, want %q", got, art)
+	}
+	one := SpriteOf(2, 1, func(x, y int) (Slot, bool) { return Good, x == 1 })
+	if got := one.Art(); got != ".g\n" {
+		t.Fatalf("SpriteOf Art() = %q", got)
+	}
+}
+
+func TestHasKnowsTofu(t *testing.T) {
+	if !Has('A') || !Has('中') || !Has('♪') || Has('拖') || Has('\u0378') {
+		t.Fatal("Has does not match the font")
+	}
+}