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.

1 files changed, 16 insertions, 0 deletions

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))