docs: rewrite README and CLAUDE.md to reflect distributed architecture

README and CLAUDE.md both described a parser + minimal query engine. The
project now includes sharding, distributed planner, remote executors,
connection pooling, 2PC transactions with durable WAL and recovery,
session auto-enlistment, window functions, CTEs, and a MySQL wire server.

Updates:
- Title/tagline: "Parser & Query Engine" -> "Parser & Distributed Query Engine"
- Architecture diagram now shows optimizer + distributed planner layers
- New sections: distributed execution, transactions, backends, tools
- Features list reorganized by subsystem (parser / engine / distributed / txn / backends)
- File layout updated with current structure (15 operators, 5 function
  categories, 3 optimizer rules, all session/shard/txn files)
- Performance and corpus numbers refreshed to the April 2026 baseline
- Test count: 1008 -> 1160 (50 test files)
- CLI examples include SSL/TLS and sharded setups
- Code samples for Session, distributed 2PC, startup recovery

Author: renecannao