feat: FiveSql2 43/43, @byref, mutable closure, RTL 479, DateTime fix

Major changes since last commit:
- FiveSql2 SQL:1999 engine (10,458 LOC) — 43/43 ALL PASS
- 21 compiler/runtime bugs fixed (short-circuit AND/OR, FOR LOOP, etc.)
- @byref pass-by-reference via RefCell pattern
- Mutable closure capture (EnsureLocalRef + RefCell sharing)
- RTL: 400 → 479 functions (+79: file, string, datetime, hash, UTF-8)
- DateTime/Timestamp fully working (hb_DateTime, hb_Hour/Min/Sec, display)
- Reserved word guard (39 keywords blocked from function calls)
- AEval arg order fix (element before index)
- Closure capture redecl fix (unique _cap_ names per block)
- Hash/string indexing in ArrayPush/ArrayPop
- Harbour compat test suite: 51/51
- 4 docs: Porting Report, Implementation Plan, Optimization Plan, Commercialization

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

_FiveSql2/test/debug_sql2.prg

@@ -0,0 +1,66 @@
+#include "FiveSqlDef.ch"
+#include "hbclass.ch"
+
+FUNCTION Main()
+   LOCAL oSql, oLexer, oParser, aTokens, hQuery, oExec
+
+   dbCreate("dbg_test", { {"ID","N",4,0}, {"NAME","C",20,0} })
+   USE "dbg_test" NEW EXCLUSIVE
+   APPEND BLANK
+   REPLACE ID WITH 1, NAME WITH "Alice"
+   APPEND BLANK
+   REPLACE ID WITH 2, NAME WITH "Bob"
+   CLOSE ALL
+
+   oLexer := TSqlLexer():New("SELECT * FROM dbg_test")
+   oLexer:Tokenize()
+   aTokens := oLexer:GetTokens()
+   oParser := TSqlParser2():New(aTokens, {})
+   hQuery := oParser:Parse()
+   oExec := TSqlExecutor():New(hQuery, {})
+
+   // Manually trace RunSelect steps
+   ? "hQuery type:", hQuery["type"]
+   ? "tables:", Len(hQuery["tables"])
+   ? "tables[1]:", ValType(hQuery["tables"][1])
+   IF ValType(hQuery["tables"][1]) == "A"
+      ? "  table name:", hQuery["tables"][1][1]
+      ? "  table alias:", hQuery["tables"][1][2]
+   ENDIF
+
+   // Trace the executor's OpenTable
+   ? "aTables:", ValType(oExec:aTables)
+   ? "oExec:hQuery:", ValType(oExec:hQuery)
+
+   // Set aTables like RunSelect does
+   oExec:aTables := hQuery["tables"]
+   ? "aTables set, len:", Len(oExec:aTables)
+
+   // Try opening table
+   LOCAL cTable, cAlias, nWA
+   cTable := oExec:aTables[1][1]
+   ? "cTable:", cTable
+
+   BEGIN SEQUENCE
+      nWA := oExec:OpenTable(cTable, "")
+      ? "OpenTable returned:", nWA
+   RECOVER
+      ? "OpenTable exception"
+   END SEQUENCE
+
+   // Try manual scan
+   IF nWA != NIL .AND. nWA > 0
+      dbSelectArea(nWA)
+      ? "FCount:", FCount()
+      dbGoTop()
+      ? "EOF:", Eof()
+      DO WHILE !Eof()
+         ? "  Row:", FieldGet(1), FieldGet(2)
+         dbSkip()
+      ENDDO
+   ENDIF
+
+   oExec:CloseOpened()
+   FErase("dbg_test.dbf")
+   ? "DONE"
+RETURN NIL