Design + build a game: from design doc to playable prototype in one conversation

Help me design and build [GAME IDEA — genre + one-sentence hook]. Target engine: [Unity / Godot / Unreal / web (Phaser or Three.js)].

STEP 1 — GAME DESIGN DOCUMENT (write this first, I\'ll approve before any code):

Core loop diagram: what does the player do every 30 seconds? Map it as a cycle: [ACTION] → [CHALLENGE] → [REWARD] → [UPGRADE/PROGRESSION] → back to action. If this loop isn\'t fun, nothing else matters.

The ONE mechanic that makes this game unique: every great game has a single mechanic that feels unlike anything else. Name it, explain why it\'s satisfying, and how it creates interesting decisions (not just reactions).

Player motivation (Bartle\'s taxonomy): is this game primarily for Achievers (mastery + completion), Explorers (discovery + secrets), Socializers (cooperation + competition), or Killers (dominance + disruption)? Design the reward systems to match.

Additional GDD sections:
- Win/lose/end conditions
- Controls (list every input and what it does)
- Difficulty curve: how does challenge escalate? (new mechanics, faster speed, smarter AI, resource scarcity)
- Art direction: one sentence on the visual style + reference games/images
- Sound design notes: what does the core action SOUND like?

STEP 2 — PROTOTYPE (the "find the fun in 48 hours" philosophy):
Build ONLY what\'s needed to test if the core loop is fun:
- Player character + movement + the ONE unique mechanic
- One challenge/enemy that tests the mechanic
- A win state and a lose state
- Placeholder art is fine — colored rectangles are better than no prototype

After each implementation step, give me a PLAYTEST CHECKPOINT:
- What to test (specific actions to try)
- What "fun" looks like (the emotion or moment I should feel)
- What to tweak if it feels off (the 2–3 variables to adjust)

STEP 3 — LAYER SYSTEMS (only after core loop is validated):
Build these one at a time, playtesting after each:
1. Level/content — a second level or scenario that introduces a twist on the core mechanic
2. Progression — score, unlocks, or difficulty scaling
3. Juice — screen shake, particles, sound effects, hit pause (frame freeze on impact)
4. UI — HUD, menus, pause screen
5. Polish — saving, settings, edge-case handling

Tip: if you can\'t describe why your game is fun in one sentence without using the word "and," you have two game ideas. Pick one. The prototype exists to answer one question: "is this fun?" — not "is this complete?"