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

src/barabadb/query/lexer.nim

@@ -75,6 +75,21 @@ type
     tkLike
     tkILike
     tkReturning
+    tkPrimary
+    tkKey
+    tkForeign
+    tkReferences
+    tkCascade
+    tkUnique
+    tkCheck
+    tkDefault
+    tkAdd
+    tkColumn
+    tkRename
+    tkBegin
+    tkCommit
+    tkRollback
+    tkExplain
     tkCount
     tkSum
     tkAvg
@@ -203,6 +218,21 @@ const keywords*: Table[string, TokenKind] = {
   "like": tkLike,
   "ilike": tkILike,
   "returning": tkReturning,
+  "primary": tkPrimary,
+  "key": tkKey,
+  "foreign": tkForeign,
+  "references": tkReferences,
+  "cascade": tkCascade,
+  "unique": tkUnique,
+  "check": tkCheck,
+  "default": tkDefault,
+  "add": tkAdd,
+  "column": tkColumn,
+  "rename": tkRename,
+  "begin": tkBegin,
+  "commit": tkCommit,
+  "rollback": tkRollback,
+  "explain": tkExplain,
   "count": tkCount,
   "sum": tkSum,
   "avg": tkAvg,