perf(fivesql2): stable temp alias — 1.67x on JOIN bench

AcquireTemp now returns the purpose string (upper-cased table name)
as the alias when available, and falls back to FA_#### only when the
same purpose is already in-flight this query — i.e., self-joins.
Previously every call returned a fresh FA_####, so the WA cache
(keyed by alias) could never hit on JOIN queries and the file got
reopened every iteration.

Bench deltas vs prior HEAD:
  B6_INNER_JOIN    217 → 130 us   -40%  (1.67x)
  B15_CTE_WIN_JOIN 1678 → 1595 us -5%
Single-table benches unchanged — they were already hitting the
cache via the table-name alias path.

B8 recursive CTE stays flat: its sub-executors at nDepth>1 still
cycle through fresh purposes that don't stabilise across queries.

FiveSql2 43/43, Harbour compat 56/56.

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

_FiveSql2/src/TSqlAlias.prg

@@ -119,11 +119,36 @@ RETURN cAlias
 
 
 /*
- * AcquireTemp — create a unique alias for temp/derived tables.
+ * AcquireTemp — create a stable alias for temp/derived tables.
+ *
+ * Returns the purpose (usually the upper-cased table name) itself
+ * when it's a valid alias and not already in use in this query.
+ * This lets the WA cache hit on repeated queries of the form
+ * `FROM table t` — previously every call returned a fresh FA_####
+ * that no cache entry could match, so every query re-opened the
+ * file. Self-joins (`FROM emp e1, emp e2`) still need two distinct
+ * workareas, so the second acquire for the same purpose falls back
+ * to the unique counter.
  */
 METHOD AcquireTemp( cPurpose ) CLASS TSqlAlias
 
-   LOCAL cAlias
+   LOCAL cAlias, cUp, i, lTaken
+
+   cUp := Upper( cPurpose )
+
+   IF ! Empty( cUp ) .AND. IsAlpha( Left( cUp, 1 ) )
+      lTaken := .F.
+      FOR i := 1 TO Len( ::aSlots )
+         IF ::aSlots[ i ][ 1 ] == cUp
+            lTaken := .T.
+            EXIT
+         ENDIF
+      NEXT
+      IF ! lTaken
+         AAdd( ::aSlots, { cUp, cPurpose, cPurpose, .F. } )
+         RETURN cUp
+      ENDIF
+   ENDIF
 
    s_nGlobalSeq++
    cAlias := "FA_" + StrZero( s_nGlobalSeq, 4 )