term: 40-column text terminal on the CGB LCD (4x8 dynamic tiles)

Add a background-tile text terminal that packs TWO 4px-wide characters
into each 8x8 tile, giving a 40x18 grid instead of the 20x18 you'd get
from an 8x8 font. The tilemap is static (one dedicated VRAM tile per
screen position); we rebuild a tile's 16 bytes from two glyphs whenever
a character changes. 360 tiles exceed the 256 a single tilemap can
address, so positions 256-359 live in VRAM bank 1 via the CGB tilemap
attribute bank-bit -- hence CGB-only.

- Switch the ROM to CGB (rgbfix -C). Safe for the FS: every process has
  PROC_WRAMB=1, so SVBK stays on bank 1 and the WRAMX FS caches don't move.
- CGB BG palette 0 = white bg / black text via BCPS/BCPD.
- src/term.asm: term_init (palette + static tilemap + clear buffer),
  build_tile (combine two 4px glyphs -> one 8x8 tile, correct VRAM bank),
  term_redraw (rebuild all tiles), term_puts, term_show, term_test.
- src/font.asm: 96-glyph 3x5-in-4x8 font, generated by tools/genfont.py.
- 40x18 text buffer at $D600 (WRAMX, above the FS caches).
- tools/ppmview.py: render a --shot PPM as ASCII so the LCD is inspectable
  from the shell during development.

Verified via emulator screenshot: "GBOS TERMINAL", the alphabet, and a
bank-1 row all render legibly at 40 columns. Serial shell + filesystem
still work unchanged.

Note: a full term_redraw builds 360 tiles and takes ~8 frames; fine for
incremental single-char updates, but scrolling needs a smarter path
(next milestone). Input (no keyboard) is also still TODO.

2 files changed, 121 insertions, 0 deletions

diff --git a/tools/genfont.py b/tools/genfont.py
new file mode 100644
index 0000000..7fe7689
--- /dev/null
+++ b/tools/genfont.py
@@ -0,0 +1,105 @@
+#!/usr/bin/env python3
+# Generate a 4x8 font (3x5 glyphs, left-aligned in a 4px cell) as RGBDS asm.
+# Output: 96 glyphs (ASCII 32..127), 8 bytes each; each byte = one 4-px row,
+# bit3=leftmost pixel of the cell, bits 3..1 = the 3px glyph, bit0 = gap.
+G = {}
+def d(ch, rows):  # rows: 5 strings of width 3 ('#'/' ')
+    G[ch] = rows
+# space
+d(' ', ["   ","   ","   ","   ","   "])
+# letters (uppercase; lowercase mapped to these)
+d('A',["## ","# #","###","# #","# #"])
+d('B',["## ","# #","## ","# #","## "])
+d('C',[" ##","#  ","#  ","#  "," ##"])
+d('D',["## ","# #","# #","# #","## "])
+d('E',["###","#  ","## ","#  ","###"])
+d('F',["###","#  ","## ","#  ","#  "])
+d('G',[" ##","#  ","# #","# #"," ##"])
+d('H',["# #","# #","###","# #","# #"])
+d('I',["###"," # "," # "," # ","###"])
+d('J',["  #","  #","  #","# #"," # "])
+d('K',["# #","# #","## ","# #","# #"])
+d('L',["#  ","#  ","#  ","#  ","###"])
+d('M',["# #","###","###","# #","# #"])
+d('N',["# #","###","###","###","# #"])
+d('O',[" # ","# #","# #","# #"," # "])
+d('P',["## ","# #","## ","#  ","#  "])
+d('Q',[" # ","# #","# #"," # ","  #"])
+d('R',["## ","# #","## ","# #","# #"])
+d('S',[" ##","#  "," # ","  #","## "])
+d('T',["###"," # "," # "," # "," # "])
+d('U',["# #","# #","# #","# #"," ##"])
+d('V',["# #","# #","# #"," # "," # "])
+d('W',["# #","# #","###","###","# #"])
+d('X',["# #","# #"," # ","# #","# #"])
+d('Y',["# #","# #"," # "," # "," # "])
+d('Z',["###","  #"," # ","#  ","###"])
+# digits
+d('0',[" # ","# #","# #","# #"," # "])
+d('1',[" # ","## "," # "," # ","###"])
+d('2',["## ","  #"," # ","#  ","###"])
+d('3',["## ","  #"," # ","  #","## "])
+d('4',["# #","# #","###","  #","  #"])
+d('5',["###","#  ","## ","  #","## "])
+d('6',[" ##","#  ","## ","# #"," # "])
+d('7',["###","  #"," # "," # "," # "])
+d('8',[" # ","# #"," # ","# #"," # "])
+d('9',[" # ","# #"," ##","  #","## "])
+# punctuation
+d('.',["   ","   ","   ","   "," # "])
+d(',',["   ","   ","   "," # ","#  "])
+d(':',["   "," # ","   "," # ","   "])
+d(';',["   "," # ","   "," # ","#  "])
+d('!',[" # "," # "," # ","   "," # "])
+d('?',["## ","  #"," # ","   "," # "])
+d('-',["   ","   ","###","   ","   "])
+d('+',["   "," # ","###"," # ","   "])
+d('=',["   ","###","   ","###","   "])
+d('*',["# #"," # ","###"," # ","# #"])
+d('/',["  #","  #"," # ","#  ","#  "])
+d('\\',["#  ","#  "," # ","  #","  #"])
+d('(',["  #"," # "," # "," # ","  #"])
+d(')',["#  "," # "," # "," # ","#  "])
+d('[',["## ","#  ","#  ","#  ","## "])
+d(']',[" ##","  #","  #","  #"," ##"])
+d('<',["  #"," # ","#  "," # ","  #"])
+d('>',["#  "," # ","  #"," # ","#  "])
+d('#',["# #","###","# #","###","# #"])
+d('$',[" # "," ##","## "," ##","## "])  # rough
+d('%',["# #","  #"," # ","#  ","# #"])
+d('&',[" # ","# #"," # ","# #"," ##"])
+d("'",[" # "," # ","   ","   ","   "])
+d('"',["# #","# #","   ","   ","   "])
+d('@',[" # ","# #","###","#  "," ##"])
+d('_',["   ","   ","   ","   ","###"])
+d('~',["   "," ##","## ","   ","   "])
+d('^',[" # ","# #","   ","   ","   "])
+d('|',[" # "," # "," # "," # "," # "])
+d('{',["  #"," # ","## "," # ","  #"])
+d('}',["#  "," # "," ##"," # ","#  "])
+d('`',["#  "," # ","   ","   ","   "])
+
+def glyph_bytes(rows):
+    out=[]
+    # 5 glyph rows in the top; cell is 4px wide, glyph 3px left-aligned (bit3..1)
+    for r in range(8):
+        if r < 5:
+            s = rows[r]
+            v = 0
+            for i,px in enumerate(s):          # i=0 leftmost -> bit3
+                if px=='#': v |= (1 << (3-i))
+            out.append(v)
+        else:
+            out.append(0)
+    return out
+
+lines=["; auto-generated by tools/genfont.py - 4x8 font (3x5 glyphs), 96 glyphs",
+       "SECTION \"font\", ROM0", "FontData::"]
+for code in range(32,128):
+    ch = chr(code)
+    key = ch.upper() if ch.isalpha() else ch
+    rows = G.get(key, G.get(ch, ["###","# #","# #","# #","###"]))  # box for unknown
+    b = glyph_bytes(rows)
+    lines.append("    db " + ",".join("$%02X"%x for x in b) + "    ; '%s'"%(ch if ch!='\\' and code!=127 else '?'))
+open("/home/user/dev/gbos/src/font.asm","w").write("\n".join(lines)+"\n")
+print("wrote src/font.inc:", (128-32), "glyphs")
diff --git a/tools/ppmview.py b/tools/ppmview.py
new file mode 100644
index 0000000..1e54bd4
--- /dev/null
+++ b/tools/ppmview.py
@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# Render a PPM screenshot as ASCII so we can "read" the GB LCD.
+import sys
+def load(path):
+    f=open(path,'rb'); assert f.readline().strip()==b'P6'
+    w,h=map(int,f.readline().split()); f.readline()
+    d=f.read(w*h*3); return w,h,d
+w,h,d=load(sys.argv[1] if len(sys.argv)>1 else 'shot.ppm')
+y0=int(sys.argv[2]) if len(sys.argv)>2 else 0
+y1=int(sys.argv[3]) if len(sys.argv)>3 else h
+for y in range(y0,min(y1,h)):
+    row=[]
+    for x in range(w):
+        i=(y*w+x)*3; b=(d[i]+d[i+1]+d[i+2])//3
+        row.append('#' if b<128 else '.')
+    print(''.join(row))