Runtime dispatch: recurrent (T=1) vs chunked (T>1) inside triton_op

Move decode/prefill dispatch inside the chunk_gated_delta_rule triton_op
instead of using torch.cond at model level. This follows the same pattern
as the SDPA triton_op (pow2/non-pow2 dispatch) and avoids torch.cond
incompatibility with AOTI's FunctionalTensor pipeline.

Changes:
- chunk_gated_delta_rule.py: Add fused recurrent Triton kernel for T=1,
  refactor chunked pipeline into _launch_chunked(), dispatch via Python
  if inside the @triton_op wrapper
- model.py: Remove torch.cond from GatedDeltaNet.forward(), call
  triton_op directly (dispatch is internal)
- export.py: Single-method export with dynamic seq_len dim
- main.cpp: Fix create_text_llm_runner API signature

Author: Gasoonjia