prototype(gc): explore static root/context binding with scoped roots

[Flamki]

Part of #68.

This is a focused follow-up to explore static root/context pairing in the API prototype without changing the merged long-lived Root<T> path.

What changed

• Added ScopedRoot<'gc, T> in examples/api_prototype/gc.rs.
• Added MutationContext::root_scoped(...) -> ScopedRoot<'gc, T>.
• ScopedRoot::get requires &MutationContext<'gc>, so usage remains tied to the same mutation lifetime.
• Added example test scoped_root_is_context_bound.
• Added UI compile-fail tests:
  • scoped_root_cannot_escape_mutate
  • scoped_root_cannot_cross_context (brand-mismatch feasibility proof)
• Updated notes/api-redesign-prototype/prototype_findings.md with the experiment rationale/results.

Why

• #68 currently uses runtime collector-id validation for long-lived roots.
• This follow-up checks how far we can push static guarantees for a scoped-root path.

Scope

• prototype/example + notes only
• no change to oscars/src collector APIs

Validation

• cargo fmt --all -- --check
• cargo test --workspace -q
• cargo clippy --workspace --all-features --all-targets -q

[jedel1043]

Duplicate of #73