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/parser/parser.go

@@ -591,6 +591,11 @@ func (p *Parser) parseClassDecl() *ast.ClassDecl {
 				p.advance() // skip keyword
 				p.advance() // skip :
 				p.skipNewlines()
+			} else if upper == "CLASSDATA" || upper == "CLASSVAR" {
+				// CLASSDATA / CLASSVAR — class-level variable (treat as DATA)
+				p.tokens[p.pos].Kind = token.DATA
+				p.current = p.tokens[p.pos]
+				members = append(members, p.parseDataDecl())
 			} else if upper == "CLASS" {
 				// CLASS VAR — class-level variable
 				p.advance() // skip CLASS
@@ -1511,6 +1516,8 @@ func (p *Parser) parseUse() *ast.UseCmd {
 	pos := p.expect(token.USE).Pos
 	var file ast.Expr
 	var via, alias string
+	var aliasExprNode ast.Expr
+	var shared, readOnly bool
 
 	// USE without args = close
 	if p.current.Kind != token.NEWLINE && p.current.Kind != token.EOF {
@@ -1520,8 +1527,22 @@ func (p *Parser) parseUse() *ast.UseCmd {
 			p.expectEndOfStmt()
 			return &ast.UseCmd{UsePos: pos}
 		}
-		file = p.parseExpr()
-		p.consumeFileExtension(file)
+		// Bare ident as filename: USE myfile / USE myfile.dbf / USE myfile NEW
+		// In Harbour, USE <name> treats name as a filename string, not a variable.
+		// Only use parseExpr for parenthesized (USE (expr)) or string literal (USE "file").
+		if p.at(token.IDENT) {
+			// Check if it's a bare filename (ident optionally followed by .ext)
+			name := p.advance().Literal
+			if p.at(token.DOT) && (p.peekAt(1) == token.IDENT || p.peekAt(1) == token.INT) {
+				p.advance() // skip DOT
+				ext := p.advance().Literal
+				name = name + "." + ext
+			}
+			file = &ast.LiteralExpr{ValuePos: pos, Kind: token.STRING, Value: name}
+		} else {
+			file = p.parseExpr()
+			p.consumeFileExtension(file)
+		}
 	}
 
 	// Parse optional clauses: VIA, ALIAS, EXCLUSIVE, SHARED, NEW, READONLY
@@ -1530,20 +1551,44 @@ func (p *Parser) parseUse() *ast.UseCmd {
 			upper := p.currentUpper()
 			if upper == "VIA" {
 				p.advance()
-				via = p.expectMethodName().Literal
+				if p.at(token.STRING) {
+					via = p.current.Literal
+					p.advance()
+				} else {
+					via = p.expectMethodName().Literal
+				}
 				continue
 			}
 			if upper == "ALIAS" {
 				p.advance()
 				if p.at(token.AMPERSAND) {
 					p.parseMacro() // macro alias — skip
+				} else if p.at(token.LPAREN) {
+					// ALIAS ( expr ) — parenthesized alias expression (runtime)
+					p.advance() // skip (
+					aliasExpr := p.parseExpr()
+					p.expect(token.RPAREN)
+					if lit, ok := aliasExpr.(*ast.LiteralExpr); ok && lit.Kind == token.STRING {
+						alias = lit.Value // constant string — store directly
+					} else {
+						aliasExprNode = aliasExpr // dynamic — evaluate at runtime
+					}
 				} else {
 					alias = p.expectMethodName().Literal
 				}
 				continue
 			}
-			if upper == "EXCLUSIVE" || upper == "SHARED" || upper == "NEW" || upper == "READONLY" ||
-				upper == "ADDITIVE" {
+			if upper == "SHARED" {
+				shared = true
+				p.advance()
+				continue
+			}
+			if upper == "READONLY" {
+				readOnly = true
+				p.advance()
+				continue
+			}
+			if upper == "EXCLUSIVE" || upper == "NEW" || upper == "ADDITIVE" {
 				p.advance()
 				continue
 			}
@@ -1557,6 +1602,16 @@ func (p *Parser) parseUse() *ast.UseCmd {
 			p.advance()
 			if p.at(token.AMPERSAND) {
 				p.parseMacro()
+			} else if p.at(token.LPAREN) {
+				// ALIAS ( expr ) — parenthesized alias expression
+				p.advance()
+				ae := p.parseExpr()
+				p.expect(token.RPAREN)
+				if lit, ok := ae.(*ast.LiteralExpr); ok && lit.Kind == token.STRING {
+					alias = lit.Value
+				} else {
+					aliasExprNode = ae
+				}
 			} else {
 				alias = p.expectMethodName().Literal
 			}
@@ -1571,7 +1626,7 @@ func (p *Parser) parseUse() *ast.UseCmd {
 	}
 
 	p.expectEndOfStmt()
-	return &ast.UseCmd{UsePos: pos, File: file, Via: via, Alias: alias}
+	return &ast.UseCmd{UsePos: pos, File: file, Via: via, Alias: alias, AliasExpr: aliasExprNode, Shared: shared, ReadOnly: readOnly}
 }
 
 func (p *Parser) parseSelect() *ast.SelectCmd {