feat: Begin browser support implementation - Phase 1

- Add comprehensive browser support plan (6-week roadmap)
- Create proof-of-concept experiments directory
- Design SQLiteAdapter interface for Node.js/browser abstraction
- Implement NodeAdapter (better-sqlite3 wrapper)
- Implement BrowserAdapter (wa-sqlite with OPFS)
- Document competitive strategy and technical decisions

Goal: Make sqlite-graph the ONLY TypeScript graph database with
ACID + algorithms + browser support (~310 KB gzipped)

Phase 1: Research & Prototype (Week 1)
Next: Build POC and validate adapter pattern

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: michaeloboyle