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>

compiler/ast/ast.go

@@ -767,10 +767,13 @@ func (s *LoopStmt) stmtNode()           {}
 
 // UseCmd represents USE [file] [VIA driver] [ALIAS name] [EXCLUSIVE|SHARED]
 type UseCmd struct {
-	UsePos   token.Position
-	File     Expr   // filename expression (nil = close current)
-	Via      string // RDD driver name
-	Alias    string // alias name
+	UsePos    token.Position
+	File      Expr   // filename expression (nil = close current)
+	Via       string // RDD driver name
+	Alias     string // alias name (static)
+	AliasExpr Expr   // alias expression for ALIAS (expr) — dynamic alias
+	Shared    bool   // SHARED flag
+	ReadOnly  bool   // READONLY flag
 }
 
 func (s *UseCmd) Pos() token.Position { return s.UsePos }