gc: add unsafe_empty_trace! parity and make trace helper macros collector-agnostic

[Flamki]

Context

• #63 still lists unsafe_empty_trace! as an open parity item.
• mark_sweep_arena2/trace.rs currently notes helper macro path coupling to mark_sweep internals.

Problem

• unsafe_empty_trace! parity is still missing against the Boa-facing boa_gc contract.
• Trace helper macros are collector-coupled, which is brittle for multi-collector work and staged Boa integration.

Proposed scope

1. Add unsafe_empty_trace! helper macro parity in Oscars trace helpers.
2. Refactor helper macro paths so they are collector-agnostic (not hardcoded to mark_sweep).
3. Keep behavior unchanged for existing empty_trace! / custom_trace! users.
4. Add focused tests/compile checks that validate helper usage from both mark_sweep and mark_sweep_arena2 contexts.

Non-goals

• No collector algorithm changes.
• No allocator/layout changes.
• No Boa integration wiring in this issue.

Acceptance criteria

• unsafe_empty_trace! exists and is usable where expected.
• Trace helper macros no longer rely on mark_sweep-only paths.
• Workspace checks pass (fmt, test, clippy; miri where applicable).
• #63 macro parity gap can be marked addressed by follow-up PR(s).