phase 8: Ouroboros boss fight — pattern puzzle with 3 victory paths

First Archon encounter: a cyclical pattern-recognition puzzle.

Boss AI: 4-phase cycle (Coil → Spray → Lash → Digest) with
escalating difficulty (10% faster per cycle, caps at 5 cycles).

Victory paths (all based on real chemistry):
- Chemical: NaOH during Spray phase (acid-base neutralization, 3x dmg)
- Direct: any projectile during Digest vulnerability window
- Catalytic: Hg poison stacks (mercury poisons catalytic sites, reduces
  regen+armor permanently)

New files: src/boss/ (types, ai, victory, arena, factory, reward),
src/data/bosses.json, src/scenes/BossArenaScene.ts, tests/boss.test.ts

Extended: ECS Boss component, CodexEntry 'boss' type, GameScene
triggers arena on Resolution phase completion.

70 new tests (455 total), all passing.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/config.ts

@@ -5,6 +5,7 @@ import { GameScene } from './scenes/GameScene';
 import { UIScene } from './scenes/UIScene';
 import { DeathScene } from './scenes/DeathScene';
 import { FractalScene } from './scenes/FractalScene';
+import { BossArenaScene } from './scenes/BossArenaScene';
 
 export const GAME_WIDTH = 1280;
 export const GAME_HEIGHT = 720;
@@ -15,7 +16,7 @@ export const gameConfig: Phaser.Types.Core.GameConfig = {
   height: GAME_HEIGHT,
   backgroundColor: '#0a0a0a',
   parent: document.body,
-  scene: [BootScene, CradleScene, GameScene, UIScene, DeathScene, FractalScene],
+  scene: [BootScene, CradleScene, GameScene, UIScene, DeathScene, FractalScene, BossArenaScene],
   physics: {
     default: 'arcade',
     arcade: {