feat: Phase 0 — pipeline integration, DDL parser, SQL executor

- Rewrote PLAN.md with 6-phase production roadmap
- Added 15 DDL/txn lexer keywords (primary, key, foreign, references, etc.)
- Added AST nodes: CreateTable, DropTable, AlterTable, BeginTxn, CommitTxn, RollbackTxn, ExplainStmt, ColumnDef
- Completed INSERT parser: VALUES, column list, RETURNING, ON CONFLICT
- Added CREATE TABLE/DROP TABLE/ALTER TABLE parsers with constraints (PK, FK, UNIQUE, NOT NULL, CHECK, DEFAULT)
- Added UPDATE/DELETE RETURNING support
- Added BEGIN, COMMIT, ROLLBACK, EXPLAIN parsers
- New query/executor.nim: AST->IR lowering + plan execution against LSM-Tree
- Wired server to executor pipeline (replaced regex-based KV INSERT)
- All 216 existing tests pass

.gitignore

@@ -11,6 +11,7 @@ benchmarks/bench_all
 benchmarks/compare
 clients/nim/tests/test_client
 src/baradadb
+src/barabadb/core/raft
 
 # Temp
 *.tmp