fix(gengo): INDEX ON ... TO (expr) evaluates filename at runtime

Prior behavior used exprToString() to serialize the TO expression
back into a string, so a runtime-evaluated filename like
`( Lower(cTable) + "_pk.ntx" )` ended up as the literal filename
`Lower(cTable) + "_pk.ntx"` on disk. Visible in FiveSql2's PRIMARY
KEY / UNIQUE DDL path: test_sql1999 was creating files with that
literal name, which the test happened not to care about because the
USE inside BEGIN SEQUENCE caught the failure.

Fix: if the File expression contains any function call (detected by
new containsCall walker), emit emitExpr + Pop2 + AsString — runtime
evaluation path. Static filenames (`TO test.ntx`) still use the
cheap exprToString branch.

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

compiler/gengo/gengo.go

@@ -617,8 +617,20 @@ func (g *Generator) emitStmt(stmt ast.Stmt, locals localMap) {
 		g.indent++
 		keyStr := exprToString(s.KeyExpr)
 		g.writeln(fmt.Sprintf("_keyExpr := %q", keyStr))
-		fileStr := exprToString(s.File)
-		g.writeln(fmt.Sprintf("_file := %q", fileStr))
+
+		// File expression: if it contains a function call, evaluate at
+		// runtime — Harbour `INDEX ON ... TO ( cExpr )` semantics. Prior
+		// behavior was static exprToString which serialized calls like
+		// `Lower(cTable) + "_pk.ntx"` into the literal filename string.
+		// Detect via containsCall; preserve static path for simple
+		// `test.ntx` style identifiers.
+		if containsCall(s.File) {
+			g.emitExpr(s.File)
+			g.writeln("_file := t.Pop2().AsString()")
+		} else {
+			fileStr := exprToString(s.File)
+			g.writeln(fmt.Sprintf("_file := %q", fileStr))
+		}
 		forExpr := `""`
 		if s.ForCond != nil {
 			forExpr = fmt.Sprintf("%q", exprToString(s.ForCond))
@@ -1783,6 +1795,26 @@ func (g *Generator) emitIndexKeyExpr(expr ast.Expr) {
 }
 
 // exprToString extracts a string representation from an AST expression.
+// containsCall reports whether an expression contains any function call.
+// Used to distinguish runtime-evaluated INDEX ON filenames
+// (`TO ( Lower(cTable) + "_pk.ntx" )`) from static ones (`TO test.ntx`).
+func containsCall(expr ast.Expr) bool {
+	if expr == nil {
+		return false
+	}
+	switch e := expr.(type) {
+	case *ast.CallExpr:
+		return true
+	case *ast.BinaryExpr:
+		return containsCall(e.Left) || containsCall(e.Right)
+	case *ast.UnaryExpr:
+		return containsCall(e.X)
+	case *ast.SendExpr:
+		return true
+	}
+	return false
+}
+
 // Used for INDEX ON key and filename, where idents are field/file names, not variables.
 func exprToString(expr ast.Expr) string {
 	switch e := expr.(type) {