Batched Cholesky stats reduction bypassed on Metal without a diagnosis (_serial_cholesky_stats / _host_reduce_cholesky_info) #138

Found while working on: T31 (PR #136)

What: for uniform batches (nbatch > 1) on Metal, cholesky_stats! (src/numeric/factorize.jl) replaces the 256-work-item _cholesky_stats_kernel! reduction with a one-work-item-per-member serial kernel (_cholesky_stats_serial_kernel!), and _numeric_info! additionally copies all of N.info to the host and recomputes info[ns + 1] per member. The PR does not record why. If the shared @localmem reduction gives a wrong info[ns + 1] on Metal, it may be a latent kernel bug (e.g. a missing barrier hidden by CUDA/AMD warp execution) that affects other backends too; if it is a Metal compiler limit, it should be named. Both hooks may not be needed (the serial kernel already writes info[ns + 1]). The host reduction also copies (ns + 1) * nb Int32 per info read on Metal.

Why it is out of scope: needs Metal hardware to reproduce; no task in TASKS.md owns it.

Suggested fix or plan change: run the batched stats kernel on Metal with SDS_TEST_ONLY=test_ubatch and both hooks off, record the failure; fix the kernel if it is a race, otherwise keep one hook and name the limit.