Coverage of `boa_gc` API surface area

[nekevss]

We need to know the exact API surface area of boa_gc as it is used in Boa.

Knowing this API surface area is necessary for us to know the API that needs to be supported by any GC in oscars.

[mrhapile]

This document is based on a deeper investigation of how the Boa engine uses the boa_gc crate. The analysis included inspecting imports, pointer usage, tracing, weak references, and interior mutability across the codebase.

The documentation intentionally provides only a concise summary of the API surface used by the engine and avoids internal implementation details of boa_gc (such as heap structures or mark-stack internals). It focuses strictly on the engine → GC interface boundary.