Add a vision-model screenshot judge to the QA tooling

`tools/screen_designs.py judge` sends each capture together with its concept
mockup to a vision model and asks whether the capture realises the same screen and
layout intent. This covers what pixel/perceptual metrics cannot judge.

- Prompt explicitly ignores art style, fidelity, fonts, and placeholder text, since
  the references are stylised mockups.
- Verdicts are classed match / review (mismatch below --fail-confidence, default
  0.6) / diff (confident mismatch, exits non-zero) / unknown; --json, --only,
  --dry-run, --model supported.
- Reads the repo .env for OPENROUTER_API_KEY without overriding real env vars.
- Default judge model google/gemini-2.5-flash-lite; openai/gpt-4o-mini verified too.

Tests/docs
- 7 new offline tests in tools/test_screen_designs.py (request payload embeds both
  images, JSON/fenced/parts/non-JSON parsing, threshold classification, .env
  loading); 30 Python tests pass.
- Live run: 3 captures match, 3 review, 0 confident diffs.
- README, TODO section moved to DONE, MEMORY. Version 0.2.15.

project.godot

@@ -8,7 +8,7 @@ config_version=5
 
 config/name="Step Line"
 config/description="Workplace security simulation about observation, access verification, and proportionate escalation."
-config/version="0.2.14"
+config/version="0.2.15"
 run/main_scene="res://scenes/main.tscn"
 config/features=PackedStringArray("4.3", "GL Compatibility")