perf(fivesql2): Go-native FetchRow fast path — 1.3-1.7x on agg/window

TSqlExecutor:FetchRow was the per-row workhorse for aggregation,
HAVING, and window queries. Even with the pre-built aFetchCache
binding columns to (nWA, nFPos), the PRG FOR loop paid one method
dispatch per column per row (dbSelectArea, FieldGet, AllTrim,
AAdd) — profile pinned it at ~30% of B4 CPU.

SqlFetchRowFast collapses the cache-path loop into a single Go
call:
  - bound entry: SelectByNum + area.GetValue directly
  - unbound (aggregate/expression): self:EvalExpr via Send
  - character values: TrimSpace inline
The PRG FetchRow keeps its original cache-miss fallback path
unchanged for rare queries where aFetchCache isn't built.

Bench deltas (median of 3 steady runs, 1000 iters):
  B4_GROUP_HAVING 418 → 327 us  -22% (1.28x)
  B9_ROW_NUMBER   191 → 120 us  -37% (1.59x)
  B10_RANK_PART   228 → 135 us  -41% (1.69x)
  B11_SUM_OVER    249 → 156 us  -37% (1.60x)
  B14_COUNT       235 → 219 us  -7%
  B15_CTE_WIN_JOIN 1577 → 1452 us  -8%
Single-table SELECT (B1-B3, B5-B7, B8) stays flat — those already
hit the column-binding fast path and don't need aggregate dispatch.

FiveSql2 43/43, Harbour compat 56/56.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

_FiveSql2/src/TSqlExecutor.prg

@@ -753,28 +753,14 @@ RETURN aCache
 METHOD FetchRow( aExprs ) CLASS TSqlExecutor
 
    LOCAL aRow := {}, i, xVal
-   LOCAL xE, cRef, nDot, nWA, nFPos, cField, cTblAlias, cA, aBound
+   LOCAL xE, cRef, nDot, nWA, nFPos, cField, cTblAlias, cA
 
-   /* Fastest path: pre-bound columns (built once per join by RunSelect) */
+   /* Fastest path: pre-bound columns (built once per join by RunSelect).
+    * Go-native: SqlFetchRowFast collapses the per-row Harbour FOR loop
+    * into a single Go call, saving ~30% of GROUP BY CPU spent in PRG
+    * method dispatch. Falls back to self:EvalExpr for unbound entries. */
    IF ::aFetchCache != NIL .AND. Len( ::aFetchCache ) == Len( aExprs )
-      FOR i := 1 TO Len( aExprs )
-         aBound := ::aFetchCache[ i ]
-         IF aBound != NIL
-            dbSelectArea( aBound[ 1 ] )
-            xVal := FieldGet( aBound[ 2 ] )
-            IF ValType( xVal ) == "C"
-               xVal := AllTrim( xVal )
-            ENDIF
-            AAdd( aRow, xVal )
-         ELSE
-            xVal := ::EvalExpr( aExprs[ i ][ 1 ] )
-            IF ValType( xVal ) == "C"
-               xVal := AllTrim( xVal )
-            ENDIF
-            AAdd( aRow, xVal )
-         ENDIF
-      NEXT
-      RETURN aRow
+      RETURN SqlFetchRowFast( Self, aExprs, ::aFetchCache )
    ENDIF
 
    FOR i := 1 TO Len( aExprs )