Explicit arguments object compatibility

Date: 2026-06-10 Area: engine

Removed the remaining implicit --compat-non-strict-mode activation of arguments objects and completed mapped arguments semantics behind the explicit --compat-arguments-object flag. The flag now controls whether the binding exists; source strictness and parameter-list shape control whether the object is mapped or unmapped, matching ES2026 §10.2.11 / §10.4.4.6 / §10.4.4.7. language.md. interpreter.md. bytecode-vm.md.