Enhancement: Import Maps from PNG/XCF Graphics

## Enhancement Goal
Add support for importing playfield maps from PNG or XCF graphics files instead of manually coding them in batariBASIC.

## Current State
- Playfields are manually defined in `Source/Data/Playfields.bas` using ASCII art patterns
- SkylineTool has `compile-2600-playfield` function but it's not fully integrated
- Makefile has commented-out rules for Map-*.png → playfield conversion
- No XCF/PNG map source files currently exist

## Proposed Implementation
1. Create XCF template files for map design in GIMP
2. Define color-to-tile mapping (e.g., red pixels = walls, blue = platforms)
3. Enhance `compile-2600-playfield` to read PNG and generate proper batariBASIC `playfield:` blocks
4. Add Makefile rules to convert Map-*.xcf → Map-*.png → playfield data
5. Allow level designers to create maps visually instead of coding ASCII

## Benefits
- Visual level design instead of text-based ASCII art
- Easier to create complex playfield layouts
- Consistent with existing XCF-based art pipeline
- Reduces manual coding errors in playfield data

## Files That Would Be Affected
- SkylineTool/src/graphics.lisp (enhance compile-2600-playfield)
- Makefile (add Map-*.xcf conversion rules)
- Source/Art/Map-*.xcf (new map source files)
- Source/Data/Playfields.bas (potentially generated instead of manual)

## Priority
Low - stretch goal enhancement beyond core functionality

## Dependencies
- SkylineTool compile-2600-playfield function (exists but may need enhancement)
- GIMP batch processing pipeline (already working for other assets)
- Color mapping specification for different tile types