[P1] Improve macOS CI lane from early guard-fail to meaningful signal #124
Description
Goal
Make macOS CI lane actionable (currently non-blocking preview due platform guard).
Scope
- Replace hard compile guard in platform layer with minimal macOS-compatible stub where feasible.
- Keep lane non-blocking until runtime parity is ready, but restore meaningful compile signal.
- Document known macOS gaps in CI workflow notes.
Acceptance Criteria
- macOS job reaches significantly further than current early platform guard failure.
- Any remaining failures are explicit and documented.