feat(cuda): bound the device block reuse cache with an env byte cap

[NicolasRouquette]

Summary

Rebased onto the updated main. Adds an opt-in byte cap on the CUDA buffer reuse cache (the
exact-size device-block free list behind take_cached_block / return_cached_block).

The cache grows without bound: a dropped buffer is returned to it, and it is only emptied on a
cudaMalloc-failure flush or at process exit. A loop over many distinct buffer sizes therefore
accretes device memory that liveBytes does not even see — a returned block is accounted as freed
before it is cached.

TORCHLEAN_CUDA_CACHE_CAP_BYTES (0 = unbounded, the prior behaviour exactly) bounds it: a returned
block that would grow the cache past the cap is freed immediately — after waiting on its completion
event, exactly as the flush path does, so an in-flight kernel never reads freed memory — instead of
being cached.

What's in it

• A running cache_bytes total under the cache mutex backs the cap decision.
• New telemetry field AllocatorStats.cacheBytes (extern torchlean_cuda_allocator_cache_bytes; the
  CPU stub returns 0); AllocatorStats.format gains cache=<MiB>.

Test

runCacheCapTest (in nn_tests_suite, runs on both the CUDA build and the CPU stub). The cap is read
once natively, so the test forks the suite binary per configuration:

• capped — a 1 MiB cap bounds an 8 MiB return workload to a 1 MiB cache;
• control — with no cap the same workload caches the full 8 MiB.

On CUDA it also checks the cap is the binding constraint (the cache fills to within one block of it).
On the stub cacheBytes stays 0, so the cap bound holds trivially.

No longer stacks on the arena PR

Rebased to stand alone on main. It previously reused a test helper from the arena PR; that helper
is now self-contained (buildCacheScratch), so this PR is a single independent commit.

Verification

CPU-stub lake build && lake exe nn_tests_suite: green (the fork test's capped / control cases
pass on the stub). The CUDA-only binding-constraint assertion requires a GPU — run
scripts/checks/check.sh --cuda.