[WIP] [NV] Add a dsr1 B300 trt agg configuration - #641

…lamp

Fixes the defect instead of stepping around it. b368eae clamped the low-latency ladder to 128
so the corrupt 256 rung was not measured; the corruption itself was still in the kernels we
publish numbers from, at roughly 1.5-3.3% per invocation on every Blackwell SKU.

The fix was already upstream. DeepEP PR #642 adds a CTA-scope fence.proxy.async.shared::cta
before mbarrier_arrive(empty_barriers[stage_idx]) in LOW_LATENCY_COMBINE_RECV, so the combine
consumer's shared-memory reads retire before the stage is declared empty and the producer's next
TMA load refills it -- signalling empty too early is what let one output row be assembled from
two tiles, which matches the observed signature exactly (norm preserved to 4 s.f., 16-40% of
elements wrong). It closed #621, the same race found independently on NVL72.

COLLX_DEEPEP_V2_COMMIT was fa8a9b16, the head of the pre-merge PR #605 branch, cut before #642
landed. Its one unique commit was the #630 single-node V2 init fix, which main carries as
56169594e, so moving to main loses nothing and also picks up #715 (system-scope release before
the GIN barrier when scale-up spans NVLink and RDMA), #688 (NCCL Device API: runtime version for
ncclDevCommCreate), #178 (SM90), #641, and #640/#627 upstream's own libnccl and SO-name handling.
Verified before bumping that main still exposes every API this adapter calls: legacy Buffer
kwargs incl. allow_mnnvl, get_low_latency_rdma_size_hint, low_latency_dispatch/combine,
ElasticBuffer's full kwarg set, topk_idx_t, and the fp8 cast helpers.

rewrite_deepep_v2 now succeeds when the source already matches, because main contains #640 and
the old count(old) == 1 assertion would have aborted every leg at repository-stage. Verified
against all four source states: old form rewrites, fixed form is a no-op, absent and duplicated
forms still fail.

_LL_LADDER_CAP returns to 256. The two constants stay separate: the receive must not be sized
from max(ladder), or clamping the ladder would change the footprint that drives transport traffic
and fp8 dequant volume. The cap test now asserts ladder <= buffer -- strict inequality encoded
the workaround, not an invariant. The backend cache key includes the pin, so this forces a
rebuild rather than reusing the old build.