docs(02-01): complete paddle2 and AI opponent plan

- Create 02-01-SUMMARY.md with task commits, decisions, and dependency graph
- Update STATE.md: position at 02-02, add AI/difficulty decisions to context
- Update ROADMAP.md: Phase 2 in progress (1/2 plans)
- Mark REQUIREMENTS.md complete: CORE-08, AI-01, AI-02, AI-03, AI-04

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

.planning/ROADMAP.md

@@ -60,7 +60,7 @@ Plans:
 5. Match ends when a player reaches target score with clear game-over state
 6. No memory leaks or event listener cleanup issues after multiple rounds
 
-**Plans:** 2 plans
+**Plans:** 1/2 plans executed
 
 Plans:
 - [ ] 02-01-PLAN.md — Paddle2, AI system (predictive interception + 3 difficulty levels), extended Input/Physics
@@ -142,7 +142,7 @@ Plans:
 | Phase | Plans Complete | Status | Completed |
 |-------|----------------|--------|-----------|
 | 1. Foundation | 1/2 | In Progress | — |
-| 2. Core Gameplay | 0/2 | Not started | — |
+| 2. Core Gameplay | 1/2 | In Progress|  |
 | 3. Complete Experience | 0/? | Not started | — |
 | 4. Polish & Depth | 0/? | Not started | — |
 | 5. Release | 0/? | Not started | — |