feat: INDEX ON with UDF support — user functions in key expressions

Core change:
- dbf.KeyEvalFunc: global callback set by gengo before OrderCreate
- evalKeyExprInner default case: calls KeyEvalFunc for unknown functions
- Final fallback: any unresolvable expression → KeyEvalFunc → MacroEval
- valueToKeyBytes: converts MacroEval result to index key bytes
- gengo: sets dbf.KeyEvalFunc = t.MacroEval before OrderCreate, clears after

Examples that now work:
  INDEX ON MyFunc(FIELD->NAME) TO idx    // UDF in key expression
  INDEX ON CityKey(FIELD->CITY, NAME) TO idx  // multi-param UDF
  INDEX ON Left(MyFunc(NAME), 15) TO idx // nested built-in + UDF

Also fixed:
- SET ORDER TO n: int→string via hbrt.NtoS (was empty string)
- CDX compound leaf decoder: proper bit-packed tag name extraction
- CDX compound recNo = direct byte offset (not page number)

All existing tests pass, NTX 47/47 + CDX 20/20 Harbour compat maintained.

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

compiler/gengo/gengo.go

@@ -482,8 +482,11 @@ func (g *Generator) emitStmt(stmt ast.Stmt, locals localMap) {
 		if s.ForCond != nil {
 			forExpr = fmt.Sprintf("%q", exprToString(s.ForCond))
 		}
+		// Set VM callback for UDF evaluation during index build
+		g.writeln("dbf.KeyEvalFunc = func(expr string) hbrt.Value { return t.MacroEval(expr) }")
 		g.writeln(fmt.Sprintf("idx.OrderCreate(hbrdd.OrderCreateParams{KeyExpr: _keyExpr, FilePath: _file, ForExpr: %s, Unique: %v, Descending: %v})",
 			forExpr, s.Unique, s.Descending))
+		g.writeln("dbf.KeyEvalFunc = nil")
 		g.indent--
 		g.writeln("}")
 		g.indent--