[Chore] Run the API tests on torchnative in CI, with capability-based skips #140

Prepare the torchnative verification of #84 (M1) and M2 before torchnative's cb-path build, #30 and TN-M1 land: a CI job that installs --extra torchnative (version as an input, default the current PyPI pre-release) and runs the same api/tests (real-model tests included), plus skip conditions that name the missing torchnative capability (e.g. continuous batching unavailable → batching tests skip with the reason; q8_0 quantisation; mps) instead of failing. On upstream PyTorch nothing may skip.

Done when: the job exists (manual + scheduled, non-blocking until torchnative is ready), the skip conditions are documented in SPEC/AGENTS, and a dry run against torchnative 0.1.0b4 shows which tests skip and why.