feat+fix(FiveSql2): window frame spec execution + EXISTS LIMIT safety

--- #12 Window frame spec now honoured ---

Parser parsed ROWS BETWEEN ... AND ... but discarded the result.
Now stores hFrame in a 6th slot on ND_WINDOW nodes via AAdd.
ApplyWindowFunctions reads it and computes per-row frame boundaries
via SqlFrameOffset helper. Unified SUM/AVG/COUNT/MIN/MAX into one
frame-aware CASE branch.

--- #6 EXISTS LIMIT mutation removed ---

Removed direct parse-tree mutation (hQuery["limit"] := 1) that
would corrupt reuse. Semi-join lift handles the fast case.

Validation: 43/43 + 51/51 + go test ALL PASS

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

_FiveSql2/src/TSqlParser2.prg

@@ -2009,7 +2009,7 @@ RETURN { aPartBy, aOrdBy, hFrame }
 /* Parse OVER(...) for window functions */
 METHOD ParseWindow( cFuncName, aFuncArgs ) CLASS TSqlParser2
 
-   LOCAL aSpec
+   LOCAL aSpec, xWinNode
 
    ::nPos++   /* eat OVER */
 
@@ -2021,8 +2021,12 @@ METHOD ParseWindow( cFuncName, aFuncArgs ) CLASS TSqlParser2
    ENDIF
 
    aSpec := ::ParseWindowSpec()
+   /* Store frame spec (aSpec[3]) in a 6th slot on the node — can't fit
+    * in the 5-slot SqlNode, so AAdd post-construction. */
+   xWinNode := SqlNode( ND_WINDOW, cFuncName, aFuncArgs, aSpec[ 1 ], aSpec[ 2 ] )
+   AAdd( xWinNode, aSpec[ 3 ] )
 
-RETURN SqlNode( ND_WINDOW, cFuncName, aFuncArgs, aSpec[ 1 ], aSpec[ 2 ] )
+RETURN xWinNode
 
 
 /* Parse frame clause: ROWS/RANGE/GROUPS BETWEEN ... AND ... (SQL:2003/2011) */