Test page: replace inverted section banners with plain '=' rules, frame page with '-'

Matches the style settled on for the print-issues receipt -- '=' for
labeled section breaks, '-' for the plain top/bottom frame, no more
fully blacked-out rows.

lib/receipt_builder.rb

@@ -65,17 +65,26 @@ class ReceiptBuilder
     self
   end
 
-  # White-on-black reverse video (GS B n) -- the closest this direct-thermal
-  # printer gets to a second "color" for a separator/banner.
+  # White-on-black reverse video (GS B n) -- available if a receipt ever
+  # wants a real highlight, but plain "=" rules read better for routine
+  # section breaks than a fully blacked-out line.
   def invert(enable = true)
     @buffer << (enable ? "\x1D\x42\x01" : "\x1D\x42\x00")
     self
   end
 
-  # A full-width inverted banner line, e.g. section_separator("SECTION").
+  # A single "=" rule with an optional label centered in it, e.g.
+  # section_separator("SECTION") -> "========= SECTION =========".
   def section_separator(label = '', width: 48)
-    padded = label.empty? ? (' ' * width) : " #{label} ".center(width)
-    feed(1).invert.text(padded[0, width]).invert(false).feed(1)
+    line = if label.empty?
+             '=' * width
+           else
+             mid = " #{label} "
+             pad = [width - mid.length, 0].max
+             left = pad / 2
+             ('=' * left) + mid + ('=' * (pad - left))
+           end
+    feed(1).text("#{line}\n").feed(1)
   end
 
   def size(mode)