[oscars-integration] Thread MutationContext through core engine and ByteCompiler

[shruti2522]

stacked on #5474

part of integrating the oscars::mark_sweep_branded GC backend. To make the GC safe at compile time using the <'gc> lifetime, we are removing MutationContext::global() and passing MutationContext down through the engine instead.

this PR threads the context through the Engine Core, Object/Shape Constructors, ByteCompiler and VM

It changes the following:

• GC Context Bridge: Made gc_collector() in boa_gc (for the oscars_backend) return a dummy static MutationContext. This acts as a safe bridge to pass the context around before we fully support closure lifetimes.
• Threaded MutationContext down from ContextBuilder::build() into Realm::create(), Intrinsics, StandardConstructors, and IntrinsicObjects.
• added _in versions for all core object and shape constructors (e.g., JsObject::new_in). These new functions take a &MutationContext. The old constructors temporarily fall back to the global context to keep the build green for standard library components.
• ByteCompiler now safely stores a reference to MutationContext (using a McWrapper to fix Debug bounds). We replaced unsafe { MutationContext::global() } in class.rs and function.rs block allocations with self.mc.0. ByteCompiler::new and its callers were updated.
• updated Vm::new to accept &MutationContext. Updated environment stack opcodes (PushScope, push_lexical, etc.) to take and use the threaded context instead of the global one.

Next

The engine now correctly passes MutationContext during setup, compilation and execution. Next PRs will update the Standard Library builtins (like Array, Date, Map) to use the new _in variants and completely remove the remaining MutationContext::global() fallbacks