Handle all-empty varlen batches

[0z5a]

Summary

• Define zero-length sequence semantics for cu_seqlens with repeated offsets.
• Avoid constructing invalid zero-extent TMA descriptors when the entire varlen batch is empty.
• Preserve initial_state in final_state, or write zeros when no initial state is supplied.

Dependency

This PR is stacked on #29. Until #29 merges, the diff includes its tile-prefix commit; after #29 lands, this PR can be rebased to contain only the empty-batch fix.

Reproduction

On the #29 baseline, mixed-empty batches (leading, middle, or consecutive empty sequences) already pass, while T_total == 0 aborts in the C++ extension when zero-extent TMA descriptors are constructed.

Validation

RTX 5080 (SM120): python -m pytest tests/test_empty_varlen.py -vv — 32 passed, covering mixed/all-empty batches, H=1/4, BF16/FP32 state, and calls with or without initial_state.