feat: Session 10.3 MCP Server + Session 11 Graph Engine Deep Integration

MCP Server (10.3):
- STDIO JSON-RPC 2.0 transport with 3 AI tools: query, vector_search, schema_inspect
- Multi-tenant session vars (tenant_id, user_id) with RLS support
- Standalone binary: build/baramcp
- Tested with all 3 tools + parameterized queries + vector search + schema inspect

Graph Engine Deep Integration (Session 11):
- CREATE GRAPH / DROP GRAPH DDL support
- Graph engine wired to SQL executor via GRAPH_TABLE() function
- 7 algorithms: BFS, DFS, PageRank, ShortestPath, Dijkstra, Louvain, Community
- INSERT into _nodes/_edges tables auto-syncs with native Graph adjacency lists
- Optional MATCH pattern, ALGORITHM, START, END, MAXDEPTH in GRAPH_TABLE syntax
- All 340+ existing tests pass

src/barabadb/query/ast.nim

@@ -41,6 +41,8 @@ type
     nkGrant
     nkRevoke
     nkSetVar
+    nkCreateGraph
+    nkDropGraph
 
     # Clauses
     nkFrom
@@ -328,6 +330,12 @@ type
     of nkSetVar:
       svName*: string
       svValue*: string
+    of nkCreateGraph:
+      cgName*: string
+      cgIfNotExists*: bool
+    of nkDropGraph:
+      dgName*: string
+      dgIfExists*: bool
     of nkApplyMigration:
       amName*: string
     of nkMigrationStatus:
@@ -440,6 +448,7 @@ type
       gtEnd*: Node
       gtMaxDepth*: int
       gtReturnCols*: seq[string]
+      gtAlgo*: string
     of nkBfsQuery:
       bfsStart*: Node
       bfsTarget*: Node