docs(gc): add oscars parity matrix for the current boa_gc API surface

[Flamki]

This PR adds a focused parity matrix for the current Boa-facing boa_gc API surface and maps it against the current oscars mark-sweep implementation.

What this adds

• docs/boa_gc_api_parity.md
  • maps the engine-facing API documented in docs/boa_gc_api_surface.md
    to current Oscars support status
  • classifies each item as:
    • implemented
    • partial
    • missing
    • semantically different
  • includes code references for the current Oscars implementation
  • calls out the main blockers for staged Boa integration
• README updates for discoverability

Why

We already have:

• the Boa-side API surface document from #38
• the integration tracker in #26
• the staged integration direction in #28

What was still missing was a concrete parity view from "what Boa uses today" to "what Oscars currently supports".
That gap matters for:

• API audit work in #26
• deciding what can be adopted incrementally in Boa
• keeping GC redesign work grounded in actual integration requirements

Scope

• docs only
• no runtime or API behavior changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace

[nekevss]

hmmmm, this looks pretty good overall, but can you open this up as an issue rather than a notes file in this instance.

I think we'll get a lot more mileage out of it, where we can create sub issues with specific tasks.

[shruti2522]

yes, opening an issue makes sense, I was also thinking about doing something similar to track API parity. This way we can keep track of the progress more easily

[Flamki]

Opened #63 to track the parity gaps in issue form as requested. Closing this PR in favor of the issue tracker entry so follow-up work can branch into smaller task-specific issues/PRs.