revert(FiveSql2): restore TSqlIndex:FindExclusive scan

Previous short-circuit (return 0 unconditionally) was a workaround
for two bugs that are both fixed now:

  1. gengo PushLocal(0) panic on unresolved identifiers
     → fixed by 08ad6f4 (PushMemvar fallback).
  2. dbInfo(DBI_FULLPATH / DBI_SHARED) returning NIL
     → fixed by d74014a (real implementations).

Restoring the original scan: walk workareas 1..250, check if any
holds an exclusive lock on the target DBF. With dbInfo now functional
and the DBI_* constants defined in include/dbinfo.ch (commit 3a00aa5),
this gives FiveSql2 real pre-flight conflict detection for concurrent
table access rather than silently proceeding into a lock failure.

Validation:
  - FiveSql2 43/43
  - standalone PRG with dbUseArea + five_SQL works (was the original
    repro that triggered the workaround)

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

_FiveSql2/src/TSqlIndex.prg

@@ -132,23 +132,34 @@ RETURN nWA
 
 METHOD FindExclusive( cTableLow ) CLASS TSqlIndex
 
-   /* Pre-flight exclusive-lock detection.
-    * Originally used dbInfo(DBI_FULLPATH)/DBI_SHARED to scan open
-    * workareas for an exclusive hold on the target DBF. In Five,
-    * `dbInfo()` is stubbed (returns NIL) and the DBI_* symbols are
-    * unresolved at compile time → runtime panic the moment any
-    * workarea is Used() when this runs (standalone PRGs routinely
-    * dbUseArea before calling five_SQL, so they tripped this).
+   /* Scan all used workareas for an exclusive hold on cTableLow.
+    * Returns the conflicting workarea number, or 0 if the table
+    * is free to open.
     *
-    * The check cannot function correctly on Five regardless, so
-    * we short-circuit to 0 (= no conflict). Matches behavior of
-    * the 43-test harness which only reaches here with no Used
-    * workareas, so the net behavior is preserved.
-    *
-    * Future: when dbInfo(DBI_FULLPATH) lands in hbrtl, restore
-    * the scan. Until then use `Alias()` + filename matching if
-    * exclusive-lock preflight becomes necessary.
+    * Fully functional now that hbrtl implements dbInfo(DBI_FULLPATH)
+    * and DBI_SHARED. The DBI_* constants resolve via include/dbinfo.ch.
     */
+   LOCAL nSaved, nArea, cDbfName, lShared
+
+   nSaved := Select()
+
+   FOR nArea := 1 TO 250
+      IF ( nArea )->( Used() )
+         dbSelectArea( nArea )
+         IF ! Empty( Alias() )
+            cDbfName := Lower( AllTrim( dbInfo( DBI_FULLPATH ) ) )
+            IF cTableLow + ".dbf" $ cDbfName .OR. cTableLow $ cDbfName
+               lShared := dbInfo( DBI_SHARED )
+               IF lShared == .F.
+                  dbSelectArea( nSaved )
+                  RETURN nArea
+               ENDIF
+            ENDIF
+         ENDIF
+      ENDIF
+   NEXT
+
+   dbSelectArea( nSaved )
 
 RETURN 0