mark_sweep: trigger deferred collection for external allocations

[Flamki]

Closes #19

This keeps the fix narrowly scoped to mark-sweep allocation accounting.

What changed

• In GcAllocator::allocate, after external/raw allocation via try_alloc_bytes(layout), we now check threshold state and set collect_needed when the heap is no longer below threshold.
• Added regression test external_allocations_set_deferred_collection_flag_when_threshold_exceeded to verify large external allocations trigger deferred collection.

Why

External allocations were bypassing deferred-collection signaling, so threshold overflow from raw allocations could go unnoticed until later operations.

Scope

• mark-sweep internals + tests only
• no public API changes
• no policy/threshold tuning changes

Validation

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-features --all-targets
• cargo clippy --workspace --no-default-features --all-targets
• cargo doc -v --document-private-items --all-features
• cargo +nightly miri test -p oscars --all-features

[nekevss]

hmmm, my one concern here is I believe the current plan is to remove the gc_allocator experiment.

See #54 for reference