Add MiniSQLite educational prototype

[xiaoruidu3]

Summary

• add a contrib/minisqlite playground that mirrors SQLite’s parser/VM/database layers on a much smaller scale
• implement a tiny SQL parser, in-memory storage engine, and execution loop behind a REPL shell
• document build instructions and provide a Makefile for compiling the standalone prototype

Testing

• cd contrib/minisqlite && make
• cd contrib/minisqlite && ./minisqlite <<'EOF' CREATE TABLE users (id INT, name TEXT); INSERT INTO users VALUES (1, Alice); SELECT * FROM users; .exit EOF

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https://chatgpt.com/codex/tasks/task_e_690a5313bce08325bf93769f78091783