fontgen - 6x6-cell font PNG to font.dat
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What it does
  Turns a font PNG (grid of 6x6 cells, 32/row = ASCII 32..) into font.dat, an
  8-byte-per-glyph table the m68k scroll text indexes at (ASCII-32)<<3:

    fontgen <font.png> <out.dat>

    byte 0      advance = ink span + 1px gap (empty cell, e.g. space = 3)
    bytes 1-6   six rows, MSB-first, glyph shifted left so its leftmost ink
                column lands on bit 7 (so the m68k shifts once to place it)
    byte 7      pad to an 8-byte stride

  Ink is any pixel differing from the top-left (background) pixel, so source
  colours don't matter.  The 192x18 asset gives 96 glyphs (ASCII 32..127).

Build
  make            (needs a C++23 compiler and libpng)
