Fix DecoupledLookback cross-block coherence (completes #91)

[shreyas-omkar]

Completes #91 — a working version of the DecoupledLookback() scan.

Background

#91 corrected DecoupledLookback()'s exclusive multi-block carries by publishing each block's inclusive aggregate in a dedicated array. That fixed the algorithm, but the cross-block publish/consume protocol was still not memory-coherent, so it failed ~40% of the time on smaller GPUs (e.g. A100 MIG 1g.5gb, sm_80) for both inclusive and exclusive scans.

Two problems remained:

1. Coherence — the aggregate was read with an ordinary (L1-cacheable) load while its companion flag was read atomically, so a consumer could observe a fresh flag == ACC_FLAG_A yet read a stale aggregate.
2. Ordering — the producer's aggregate store was not ordered before its flag store (nor the consumer's flag load before its aggregate load) at device scope.

Fix

• Coherence: the aggregate is now read/written with a relaxed (monotonic), L1-bypassing atomic, reinterpreted through a same-width unsigned integer so float element types are supported (compile-time fallback to a plain access for other widths).
• Ordering: a device-scope memory fence now separates the aggregate store from the flag store (producer) and the flag load from the aggregate load (consumer).

There is no vendor-agnostic device-scope fence in the ecosystem today (KA only has work-group barrier/@synchronize; UnsafeAtomics.fence(acq_rel) does not lower on NVPTX; CUDA.threadfence is CUDA-only). So _decoupled_fence() composes the already-available primitives, kept self-contained here:

• generic definition = UnsafeAtomics.fence(acq_rel) (lowers on OpenCL/SPIR-V, Metal, oneAPI, AMDGPU),
• overridden to the native CUDA.threadfence() (membar.gl) via a small CUDA package extension, since NVPTX does not select scoped atomic fences.

No new hard dependency: CUDA is a weakdep behind an extension.

Also: aggregates is now always its own contiguous array (never a temp view) so pointer(aggregates, i) is valid inside the kernel; the non-uniform exclusive-scan test is re-extended to cover DecoupledLookback().

Validation

• CUDA (RTX 5080): 4400/4400 passed (exclusive + inclusive, random sizes/block-sizes, incl. Float32/Float64) — 0 failures.

Performance / when to use it

DecoupledLookback() is now correct, but it is not the fastest scan on CUDA — launches are cheap there, so the standalone ScanPrefixes() (which stays the default on every backend) wins, and the gap grows with n:

n (Int32, excl.)	DecoupledLookback	ScanPrefixes	DL / SP
10 k	0.091 ms	0.097 ms	0.94×
100 k	0.120 ms	0.103 ms	1.17×
1 M	0.292 ms	0.111 ms	2.62×
4 M	0.910 ms	0.171 ms	5.33×
16 M	3.15 ms	0.31 ms	10.1×

So this PR does not change the default (ScanPrefixes() everywhere). Its value is making DecoupledLookback() correct so it can be opted into on dispatch-bound backends (e.g. Metal), where collapsing kernel launches is the win — the motivation for the single-pass/onesweep direction.