docs: add success criteria and product scope to PRD

_bmad-output/planning-artifacts/prd.md

@@ -1,5 +1,5 @@
 ---
-stepsCompleted: ['step-01-init', 'step-02-discovery', 'step-02b-vision', 'step-02c-executive-summary']
+stepsCompleted: ['step-01-init', 'step-02-discovery', 'step-02b-vision', 'step-02c-executive-summary', 'step-03-success']
 inputDocuments:
   - '_bmad-output/brainstorming/brainstorming-session-2026-04-12-001.md'
 workflowType: 'prd'
@@ -30,3 +30,63 @@ This project prioritizes craft over novelty. Rather than adding twists or unique
 - **Complexity:** Low — single-player, no backend, no auth, localStorage only
 - **Project Context:** Greenfield — new project from scratch
 - **Tech Stack:** Svelte + Vanilla JS + Tailwind CSS + Vite + GitHub Pages
+
+## Success Criteria
+
+### User Success
+
+- Player can slide tiles in all 4 directions with responsive, satisfying feedback
+- Merge mechanics match the original exactly (leading-edge order, once-per-move rule)
+- Tile spawning follows the original's 90/10 probability split
+- Game feels smooth — animations at original timing (100ms slide, 200ms pop/bounce)
+- Score and best score tracked accurately across sessions
+- Playable on both desktop (keyboard) and mobile (touch/swipe)
+
+### Business Success
+
+Not applicable — this is a personal exploration/learning project with no business metrics. Success is measured by:
+- Completing a fully functional 2048 clone
+- Learning Svelte, Tailwind, and Vite through hands-on building
+- Deploying a playable game to GitHub Pages
+
+### Technical Success
+
+- Clean Svelte component architecture (App, Grid, Tile, ScoreBoard, GameMessage)
+- Game logic separated from UI (pure JS module, testable independently)
+- All CSS animations matching original timings
+- Responsive layout (500px desktop, 280px mobile)
+- localStorage persistence working correctly (game state + best score)
+
+### Measurable Outcomes
+
+- All original 2048 mechanics faithfully reproduced (merge rules, spawn rules, win/lose detection)
+- Game playable end-to-end: start → play → win/lose → restart
+- "Keep playing" mode works after reaching 2048
+- No visual glitches during slide/merge/spawn animations
+- Game state survives page refresh
+
+## Product Scope
+
+### MVP - Minimum Viable Product
+
+- 4x4 grid with tile sliding and merging
+- Score and best score display
+- Keyboard input (arrows)
+- Win/lose detection with overlays
+- New Game button
+- Basic tile colors matching original
+
+### Growth Features (Post-MVP)
+
+- Touch/swipe input for mobile
+- Full animation suite (slide, pop, bounce, score float)
+- localStorage persistence (game state + best score)
+- Responsive design (desktop + mobile)
+- WASD and Vim key support
+- "Keep playing" mode after winning
+
+### Vision (Future)
+
+- Deploy to GitHub Pages
+- Tile glow effects on 128+ values
+- Polish: exact color matching, font sizing tiers, overlay fade timing