feat(sql): Window Functions + MERGE statement + REST bridge plan

- Add Window Functions: ROW_NUMBER, RANK, DENSE_RANK, LEAD, LAG,
  FIRST_VALUE, LAST_VALUE, NTILE with PARTITION BY, ORDER BY,
  ROWS/RANGE frame specifications
- Add MERGE/UPSERT statement with WHEN MATCHED UPDATE and
  WHEN NOT MATCHED INSERT
- Add SQL/PGQ Property Graph long-term plan in PLAN_SQL_ADVANCED.md
- Add 7 new tests for Window Functions and MERGE
- Update baraql.md documentation

src/barabadb/query/lexer.nim

@@ -120,6 +120,8 @@ type
     tkAvg
     tkMin
     tkMax
+    tkMerge
+    tkMatched
     tkArray
     tkVector
     tkGraph
@@ -134,6 +136,24 @@ type
     tkDfs
     tkPath
 
+    # Window functions
+    tkOver
+    tkPartition
+    tkRows
+    tkRange
+    tkUnbounded
+    tkPreceding
+    tkFollowing
+    tkCurrent
+    tkRowNumber
+    tkRank
+    tkDenseRank
+    tkLead
+    tkLag
+    tkFirstValue
+    tkLastValue
+    tkNtile
+
     # Operators
     tkPlus
     tkMinus
@@ -291,6 +311,8 @@ const keywords*: Table[string, TokenKind] = {
   "avg": tkAvg,
   "min": tkMin,
   "max": tkMax,
+  "merge": tkMerge,
+  "matched": tkMatched,
   "array": tkArray,
   "vector": tkVector,
   "graph": tkGraph,
@@ -301,6 +323,22 @@ const keywords*: Table[string, TokenKind] = {
   "bfs": tkBfs,
   "dfs": tkDfs,
   "path": tkPath,
+  "over": tkOver,
+  "partition": tkPartition,
+  "rows": tkRows,
+  "range": tkRange,
+  "unbounded": tkUnbounded,
+  "preceding": tkPreceding,
+  "following": tkFollowing,
+  "current": tkCurrent,
+  "row_number": tkRowNumber,
+  "rank": tkRank,
+  "dense_rank": tkDenseRank,
+  "lead": tkLead,
+  "lag": tkLag,
+  "first_value": tkFirstValue,
+  "last_value": tkLastValue,
+  "ntile": tkNtile,
 }.toTable
 
 proc newLexer*(input: string): Lexer =