perf(fivesql2): gate CTE __cte_*.dbf cleanup on legacy disk fallback

CTE tables now materialise via MEMRDD (no file on disk), yet the
RunSelect cleanup loop was still stat-ing __cte_<name>.dbf for every
CTE in every CTE query. Profile after the FetchRow rewrite pinned
HbFileExists at 20.28% of total CPU — pure waste when MEMRDD is the
common path.

Add s_lCteDiskSeen flag, set only when the legacy DBFNTX fallback in
RunSelect actually opens a pre-existing __cte_<name>.dbf (line 1247
path — rare, only for sub-executors referencing a CTE by name on a
crashed-prior-run .dbf). Cleanup runs only when the flag is set.

pprof delta (full bench with cache enabled):
  rawsyscalln: 25.56% → 8.50%  (~17 points removed)
  HbFileExists: 20.28% → 0%    (dropped out of top)
Wall-clock unchanged (ENOENT stats are kernel-cached on Darwin), but
this removes the last visible avoidable syscall. What's left in the
profile (kevent, madvise, pthread_cond_*) is Go runtime + scheduler
overhead that application code can't touch.

FiveSql2 43/43, Harbour compat 56/56.

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

_FiveSql2/src/TSqlExecutor.prg

@@ -18,6 +18,11 @@
 STATIC s_aOuterStack := {}
 STATIC s_hAutoInc := NIL
 STATIC s_nRCJSeq := 0
+/* Set .T. the first time CTE cleanup sees a legacy __cte_<name>.dbf
+ * file on disk, or the legacy DBFNTX open path fires. Profile showed
+ * the stat loop at ~20% of total CPU otherwise — MEMRDD is the norm
+ * for CTEs so the .dbf doesn't exist and the stat is pure overhead. */
+STATIC s_lCteDiskSeen := .F.
 
 /* Per-plan DML pcode cache. Keyed by the plan-cache key that TFiveSQL
  * uses (template key or cSQL text); value is a hash:
@@ -1245,6 +1250,7 @@ METHOD RunSelect() CLASS TSqlExecutor
                nWA := 0
             END SEQUENCE
             IF nWA == 0 .AND. hb_FileExists( "__cte_" + Lower( cTable ) + ".dbf" )
+               s_lCteDiskSeen := .T.
                BEGIN SEQUENCE
                   dbUseArea( .T., "DBFNTX", "__cte_" + Lower( cTable ) + ".dbf", ;
                      cAlias, .T., .T. )
@@ -1640,9 +1646,14 @@ METHOD RunSelect() CLASS TSqlExecutor
             dbSelectArea( nWA )
             dbCloseArea()
          ENDIF
-         cTable := "__cte_" + Lower( ::hQuery[ "cte" ][ i ][ 1 ] )
-         IF hb_FileExists( cTable + ".dbf" )
-            FErase( cTable + ".dbf" )
+         /* Legacy disk fallback cleanup — only runs when a __cte_*.dbf
+          * has actually been seen (either from a prior crash or a
+          * MEMRDD-failure legacy open). MEMRDD-only runs skip the stat. */
+         IF s_lCteDiskSeen
+            cTable := "__cte_" + Lower( ::hQuery[ "cte" ][ i ][ 1 ] )
+            IF hb_FileExists( cTable + ".dbf" )
+               FErase( cTable + ".dbf" )
+            ENDIF
          ENDIF
       NEXT
    ENDIF