feat(sql): Vector SQL Integration + test isolation fixes

- Add VECTOR(n) column type support in CREATE TABLE
- Add CREATE INDEX ... USING hnsw/ivfpq for vector indexes
- Add cosine_distance(), euclidean_distance(), inner_product(), l1/l2_distance()
  SQL functions in expression evaluator
- Add <-> nearest-neighbor operator
- Fix ORDER BY with non-projected columns (move irpkSort before irpkProject)
- Fix execInsert to escape comma-containing values (vector literals)
- Fix MERGE tests by using unique temp dirs per test suite
- Add 8 Vector SQL Integration tests (all passing)
- Update PLAN_SQL_ADVANCED.md

src/barabadb/query/lexer.nim

@@ -204,6 +204,7 @@ type
     tkConcat
     tkCoalesce
     tkFloorDiv
+    tkDistanceOp   # <->
     tkPlaceholder
 
     # Special
@@ -572,6 +573,11 @@ proc nextToken*(l: var Lexer): Token =
     discard l.advance()
     return Token(kind: tkInvalid, value: "!", line: startLine, col: startCol)
   of '<':
+    if l.pos + 2 < l.input.len and l.input[l.pos + 1] == '-' and l.input[l.pos + 2] == '>':
+      discard l.advance()
+      discard l.advance()
+      discard l.advance()
+      return Token(kind: tkDistanceOp, value: "<->", line: startLine, col: startCol)
     if l.pos + 1 < l.input.len and l.input[l.pos + 1] == '=':
       discard l.advance()
       discard l.advance()