Reuse tile prefix in recurrence kernel

[0z5a]

Summary

• Pass the varlen tile prefix built before kernel1 to kernel2.
• Replace kernel2's per-CTA linear scan of preceding sequence lengths with a direct tile_prefix[seq_idx] lookup.
• Keep the fixed-length path, workspace size, and launch count unchanged.

Validation

• Built on RTX 5080 (SM120).
• Exact Torch-reference tests: fixed length and ragged H=96.
• 8/8 N=8 varlen state/dtype combinations passed.
• N=256, T=8192, H=16: kernel2 563.84 us -> 502.66 us (10.9%); full forward median 0.782 ms -> 0.731 ms (6.5%).
• N=1 full-forward median: 1.721 ms -> 1.720 ms (no regression).

Notes

Zero-length sequences are an existing unsupported input: they hang on the baseline too. This focused optimization does not change that behavior.