jax.jit / torch.compile tests + fix torch detected mode (delegate detection to array-api-compat)

[egparedes]

Adds framework-compilation tests for generated code — and fixes the real bug they uncovered: torch tensors never worked in detected mode.

The bug

The generated preamble filtered arguments with hasattr(a, '__array_namespace__'), but torch.Tensor has never exposed that attribute in any 2.x release — torch tensors were dropped like scalars and array_namespace() raised TypeError. The same homegrown duck-check also broke verify=, the fallback wrapper, and closure captures for torch. This contradicted the README/docs claims of torch support; it was invisible because torch was never installed in CI.

The fix — delegate ALL array detection to array-api-compat

• Generated preamble: xp = array_namespace(x, y) — arguments passed straight through (compat ignores Python scalars/None since 1.10 and dispatches torch/CuPy). Floor bumped to array-api-compat>=1.10; 41 goldens regenerated (namespace line only).
• _is_array (verify + fallback): fully delegated to array_namespace via try/except — no homegrown partial implementations of the standard's detection machinery.
• Closure classification (extract.py): captured torch tensors now become hidden closure params (previously misclassified). Verified no numpy reclassifications occur.
• Bonus fix: verify= built its expected side with xp.asarray(list), which torch defaults to float32 — now explicit dtype=xp.float64 (backend-neutral).

Tests (93, zero skips in the compile job)

• 88 compilation tests: 22 corpus functions (branches, loops, closures, helpers, kw-only defaults) × jax.jit detected/pinned × torch.compile detected/pinned, compared against eager results.
• 5 detection regression tests: verify-with-torch, fallback-with-torch, pinned fallback, closure-captured torch tensor, jax closure twin.

CI / infra

• New compile job (CPU-only torch wheel, jax; import guard so skips can't hide failures), make backends target, backends optional dependency group, docs note (torch.compile of detected-mode functions needs compat >= 1.15).

Verification

93/93 framework tests (jax 0.11.2, torch 2.14.1+cpu); make check 804 passed / 96% coverage; make docs clean; compat==1.10 floor verified green (711 passed, one over-specified error-message regex widened). Zero graph breaks in torch.compile pinned mode.