Strict-only test262 handling

Date: 2026-05-17 Area: testing Pull Request: #645

The test262 runner now injects a "use strict" directive for onlyStrict tests while still passing --compat-non-strict-mode to Script tests. This lets strict-only coverage exercise strict arguments, with, assignment, delete, and this semantics without losing the parser/runtime support needed for non-strict Script tests. test262.md.