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/src/TFiveSQL.prg

@@ -0,0 +1,66 @@
+/*
+ * TFiveSQL.prg — Main facade class for FiveSql2 engine
+ *
+ * Uses TSqlParser2 (Pratt parser) exclusively.
+ *
+ * FiveSql2 — SQL Engine for Harbour DBF/NTX
+ *
+ * Copyright (c) 2025-2026 Charles KWON (Charles KWON OhJun)
+ * Email: charleskwonohjun@gmail.com
+ *
+ * All rights reserved.
+ */
+
+#include "hbclass.ch"
+#include "FiveSqlDef.ch"
+
+CLASS TFiveSQL
+
+   DATA oLexer
+   DATA oParser
+   DATA oExec
+   DATA aParams INIT {}
+
+   METHOD New( aParams ) CONSTRUCTOR
+   METHOD Execute( cSQL )
+   METHOD ExecuteWith( cSQL, aParams )
+
+ENDCLASS
+
+
+METHOD New( aParams ) CLASS TFiveSQL
+
+   IF aParams != NIL
+      ::aParams := aParams
+   ENDIF
+
+RETURN SELF
+
+
+METHOD Execute( cSQL ) CLASS TFiveSQL
+
+   LOCAL aTokens, hQuery, aResult
+
+   /* Parse — no caching (plan trees are mutated during execution) */
+   ::oLexer := TSqlLexer():New( cSQL )
+   ::oLexer:Tokenize()
+   aTokens := ::oLexer:GetTokens()
+
+   ::oParser := TSqlParser2():New( aTokens, ::aParams )
+   hQuery := ::oParser:Parse()
+
+   IF hQuery == NIL
+      RETURN { { "__error__" }, { { SQL_ERR_SYNTAX, "Failed to parse SQL", cSQL } } }
+   ENDIF
+
+   ::oExec := TSqlExecutor():New( hQuery, ::aParams )
+   aResult := ::oExec:Run()
+
+RETURN aResult
+
+
+METHOD ExecuteWith( cSQL, aParams ) CLASS TFiveSQL
+
+   ::aParams := aParams
+
+RETURN ::Execute( cSQL )