optimize qwen3.8-flash-next - #185

main landed the same feature independently in modelscope#174, so qwen4_exp.py /
ple.py / qsa_indexer.py / hyper_connection_gated.py came out as add/add
conflicts. Resolved to this branch's versions -- they are supersets:
  select_mask              -> selection_as_mask (+ selection_as_token_indices,
                              select_token_indices_thd for the sparse kernel)
  _set_ple_ngram_embedding -> fill_table_from_hf / export_table_to_hf
                              (offload-aware, and reduces the offload flag
                              across pp before gating pp collectives)
  _warn_qsa_fallback_once  -> dropped with the fallback path itself

Kept main's ple_seed instead of this branch's hardcoded _PLE_SEED: the
parser derives it from text_config.seed (defaulting to 1234), which stays
configurable and still avoids the vLLM config-pollution issue. Wired it
through Qwen4ExpTextNGramEmbedding with the same fail-loud treatment as
eos_token_id / split_ngram_parts.

Also picks up modelscope#175 (packed sequence length handling for mcore 0.16.0/0.16.1).