Phase 1: asset pipeline (Python script + 5 processed .bin/.attr files)

Adds tools/convert_screens.py which converts each source PNG to a
160x200 multicolor bitmap (8000 B .bin) plus a 1000 B .attr screen-
memory table, following c64-wiki Multicolor_Bitmap_Mode:

  - 2-bit pixel: 00=$D021, 01=attr high nibble, 10=attr low nibble,
    11=color RAM nibble.
  - Per 4x8 cell: $D021 is forced into the 4-color set (it's global);
    the other 3 are the 3 most common non-d021 colors in the cell.
  - Pixels snap to the nearest of those 4, then packed 4-per-byte
    MSB-first into the bitmap; attr byte = (cell_color_2 << 4) |
    cell_color_1 (low nibble = '10' color, high = '01' color).

Tried two downscale strategies on all 5 source images; both produce
visually equivalent output at the ~9x source-to-target scale. Picked
Strategy A (direct 160x200 LANCZOS) as default because it's simpler
and slightly faster; Strategy B (LANCZOS 320x200 then 2x BOX 160x200)
remains available via --strategy. See the script docstring for the
detailed rationale.

$D021 is auto-picked as the most common C64 palette color in the
downscaled image (turns out to be black for 4 of 5 screens, dark grey
for waiting2). The .d021 sidecar files document this value so Phase 2
can program the VIC without re-deriving it.

The '11' color (color RAM nibble per cell) is not stored in .attr (no
room in 1000 B). Default is to leave color RAM at the C64 boot value
(black), which gives effectively 3 unique colors per cell + 1 global.
Phase 2 can either accept this or extend the format with a per-cell
color RAM table.

Generated files are checked in so the build doesn't depend on Python.

Output sizes verified: each .bin is exactly 8000 B, each .attr is
exactly 1000 B, total 9000 B per screen. The .bin files are not all
zero (sanity check passed).

.gitignore

@@ -11,10 +11,20 @@ src/build/
 *.bin
 *.crt
 
+# but DO commit the asset-pipeline outputs (see tools/convert_screens.py)
+!src/data/processed/*.bin
+!src/data/processed/*.attr
+!src/data/processed/*.d021
+!src/data/raw/
+
 # oscar64 build output (built by `make -C make compiler` from oscar64 source)
 oscar64/bin/
 oscar64/build/
 
+# python cache
+__pycache__/
+*.pyc
+
 # editor / IDE
 .vscode/
 .idea/