fix: P0 CI unblock + P2 pixel_artist.py split (2004->99 lines)

[lulin70]

Summary

P0 CI unblock + P2 large file split — dual-track fix.

P0: CI Pipeline Unblock (2 jobs were FAILING)

Job 1: Lint & Type Check — FIXED

• 6 mypy import-not-found errors from ghost/incorrect import paths:
  • commander_ai.py: domain.combat.combat_engagement -> domain.ai.combat_engagement
  • commander_ai.py: domain.difficulty.difficulty_system -> domain.ai.difficulty_system
  • effect_renderer.py + sprite_renderer.py: relative vec2 import -> absolute
  • terrain_rendering_system.py: domain.entities.enhanced_tile -> domain.systems.enhanced_tile
• Added scipy.*, py7zr, AppKit to mypy ignore_missing_imports
• Result: mypy errors reduced from 9 -> 1 (union-attr, non-blocking in CI)

Job 2: docker-build — FIXED

• Root cause: pip install ran before COPY src/, so egg_base='src' failed
• Fix: Reordered Dockerfile COPY before RUN
• Also added --no-build-isolation fallback (from PR fix: P1-P2 large file split + security/hygiene cleanup -- Phase 3 #10)

PR #6 merged: dependabot actions/checkout v6->v7

P2: pixel_artist.py Split (2004 -> 99 lines, -95%)

New File	Content	Lines
pixel_canvas.py	CCPalette, PaletteSet, PixelCanvas, add_noise, dither_pattern	309
unit_sprite_generator.py	UnitSpriteSpec, UnitSpriteGenerator (6 unit draw methods)	332
terrain_tile_generator.py	TerrainTileGenerator (20 terrain type generators)	1315
pixel_artist.py (rewritten)	Re-exports + 2 factory functions	99

Backward compatibility: All existing imports of create_unit_sprite and create_terrain_tile work unchanged.

Test Results

4298 passed, 25 skipped, ruff All checks passed

Files Changed

• Modified (9): Dockerfile, pyproject.toml, commander_ai.py, effect_renderer.py, pixel_artist.py, pixvoxel_loader.py, sprite_renderer.py, terrain_rendering_system.py, window_config.py
• New (3): pixel_canvas.py, unit_sprite_generator.py, terrain_tile_generator.py