fix: use 64-bit mega moe sf offsets

[netaddi]

Summary

Fix SM100 Mega MoE scale-factor pool address arithmetic for large
num_max_tokens_per_rank.

This PR:

• uses 64-bit arithmetic for SM100 Mega MoE SF-pool offsets;
• adds a manual SM100 capacity regression test;
• excludes the heavy multi-process regression from sanitizer auto-discovery.

Bug Discovery

While validating Mega MoE in a serving-like SM100 setup, the output changed when
only the symmetric-memory buffer capacity changed.

The reproducer uses identical live tokens, inputs, routing, and weights, then
runs:

• small capacity: num_max_tokens_per_rank=32
• large capacity: num_max_tokens_per_rank=200000

The capacity should only affect allocation size. It should not affect the
mathematical output for the same live tokens.

Root Cause

In sm100_fp8_fp4_mega_moe_impl, some SF-pool offsets were computed with
32-bit intermediate values.

For large num_max_tokens_per_rank, kNumPaddedSFPoolTokens becomes large
enough that expressions like:

j * kNumPaddedSFPoolTokens + sf_pool_token_idx

and:

k_uint_idx * mn_stride + sf_pool_token_idx * sizeof(uint32_t) + byte_idx

can overflow 32-bit arithmetic before being used as addresses.

That makes the kernel write or read FP8/FP4 scale factors at the wrong SF-pool
locations, so the subsequent GEMMs consume incorrect scales and produce wrong
outputs.

Fix

Promote the SF-pool offset calculations to uint64_t in both affected paths:

• dispatch-side local SF pool write;
• L2 SF buffer address calculation.

This keeps the indexing valid for large Mega MoE capacities while preserving the
existing layout and kernel behavior for normal capacities.

Validation

Syntax / diff checks:

python3 -m py_compile tests/test_mega_moe_capacity.py tests/test_sanitizer.py
git diff --check

Manual SM100 regression test:

python tests/test_mega_moe_capacity.py \
--num-processes 8 \
--small-capacity 32 \
--large-capacity 200000

Expected result after the fix:

• all ranks produce matching small/large output hashes;
• max_abs_diff == 0.0;
• mean_abs_diff == 0.0.