implement artifact win condition (REQ-WIN-ARTIFACT-COUNT, REQ-WIN-SCREEN)

- Add [artifacts] config section to world.toml with artifact_chance_formula (x = station level) and artifact_win_count
- Roll artifact chance before each schematic drop; on success show 2 schematics + Artifact option instead of 3
- Selecting Artifact increments artifact count; reaching artifact_win_count triggers win screen
- Display "Artifacts: x/y" in header bar via ArtifactCountChangedEvent
- Win screen mirrors game-over screen with "Won!" caption and same Restart/Quit buttons

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DUsFgd2Ga6pmLz8giS8WUn

bin/test/data/config/world.toml

@@ -29,6 +29,10 @@ target_score_formula      = "1 / (1 + x)"          # x = distance / max weapon r
 overclaim_penalty_formula = "max(0.5, 1 - 0.1*x)"  # x = competing claim count; multiplies score, clamped to [0,1]
 target_hysteresis         = 0.10                   # keep current target unless a challenger beats it by >10%
 
+[artifacts]
+artifact_chance_formula = "0.05 * x"  # 5% chance per station level
+artifact_win_count      = 3
+
 [waves]
 threat_rate_formula = "x"
 ship_level_formula = "1 + x / 10"