feat: retro mode — arcade combo round with gamepad overlays

Every fight now includes one Retro Mode round where bots submit
gamepad combo inputs (↑↓←→ A B). 24 moves across 4 tiers: basic
(always shown), standard (partially revealed), super (must discover),
and ultra (KONAMI CODE for 50 dmg). Discovery bonus gives 1.5x damage.

Includes pixel-art gamepad overlays (P1/P2) with animated button
presses, retro-specific narrations, and mock bot combo responses
scaled by ELO.

Also adds loops/plan.md with 11-phase production hardening roadmap.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/mock.ts

@@ -3,6 +3,7 @@ import { createHash, randomBytes } from 'crypto'
 import { db, schema, sqlite } from '../db/index.js'
 import { randomArena } from './arenas.js'
 import { pickChallenge, type Challenge } from './challenges.js'
+import { generateMockRetroResponse } from './retro-moves.js'
 import { scoreRound, calculateElo, calculateTier } from './scoring.js'
 import { eq, sql } from 'drizzle-orm'