fix(kernels): drop the D2H sync from the varlen GDN/KDA prefill conv

[dejay2]

What

causal_conv1d_varlen only needs the longest request in the batch to size its
triton launch grid, but the only place that number lived was on the device, so the
triton fallback derived it with int(seq_lens.max().item()).

build_fla_metadata already computes the per-request lengths on the host, so it
now carries FLAMetadata.max_seq_len, and the three linear-attention ops that call
the conv (qwen3_5_moe/gdn.py, qwen4_exp/gdn.py, glm5_next/kda.py) pass it
down. The new kwarg is optional; with it omitted the kernel wrapper derives the
value on device exactly as before.

Why

int(seq_lens.max().item()) is a device-to-host sync on every prefill: the whole
pipeline stalls to read back a number the scheduler computed on the host in the
first place. It is also illegal inside a CUDA stream capture, so its presence alone
makes the prefill forward of every GDN/KDA model uncapturable. Passing the host
value removes both problems without touching the kernel.

How it was tested

Windows 11, RTX 5090, triton fallback path (no sgl_kernel installed).

python -m pytest -q \
  tests/kernels/test_causal_conv1d_capture.py \
  tests/models/qwen4_exp/test_gdn.py \
  tests/models/test_glm5_next_kda_snapshot.py \
  tests/models/test_glm5_next_kda_op.py \
  tests/kvcache/test_linear_state_pool_alloc.py

• before this commit: 21 passed
• after this commit: 26 passed (5 new)

The new tests/kernels/test_causal_conv1d_capture.py pins that the host-metadata
path performs no .item() at all, that the default device-derived path still
works, that both produce bit-identical output and conv-state updates, and that the
call captures into and replays from a torch.cuda.CUDAGraph.

What is NOT included

• Nothing in kernel/triton/causal_conv1d_triton.py: it already accepts an optional
  max_seq_len and falls back to the device-side max when it is None. This PR
  only supplies the value.
• The fork branch this comes from also primes the fla chunk-index cache before
  graph capture. That helper has no caller outside the fork's speculative-decoding
  graph runner, so it is left out here.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RG8BXfsSZi1nh4wMZnhJQK