perf(parser): scan the input as bytes instead of runes

Assisted-by: GLM 5.3

interpres.go

@@ -110,7 +110,9 @@ func ParseContext(ctx context.Context, data []byte) (map[string]any, error) {
 	if !utf8.Valid(data) {
 		return nil, &SyntaxError{Line: 1, Msg: "input is not valid UTF-8"}
 	}
-	p := &parser{src: []rune(string(data)), line: 1, ctx: ctx}
+	// The parser scans data in place; it only reads the buffer, and every
+	// string it stores in the tree is copied out of it.
+	p := &parser{src: data, line: 1, ctx: ctx}
 	return p.parse()
 }