feat: add onerec worker impl for rec framework[5/8].

[DragonFive]

This PR is a follow-up to #565 and continues the generative recommendation work by introducing the runtime rec worker
layer for the OneRec execution path.

What’s included

• Runtime worker implementation for rec requests:
  • RecWorkerImpl (abstract base): shared model init + rec input preparation via
    Batch::prepare_rec_forward_input(...)
  • OneRecWorkerImpl (concrete): executes the OneRec prefill flow
• WorkerType::REC wiring: Worker now instantiates OneRecWorkerImpl (removes the temporary LLMWorkerImpl fallback)
• Two-stage first-prefill execution:
  • encoder forward (from rec_params.encoder_sparse_embedding or rec_params.encoder_token_ids)
  • decoder forward
  • subsequent steps run decoder-only
• Driver-only sampling/output: non-driver ranks return std::nullopt (same contract as the LLM path)
• Tunable input-builder thread pool via XLLM_REC_INPUT_BUILDER_THREADS (driver only)

Out of scope (planned follow-ups)

• Input/output processors (request ingestion and result emission)
• Concrete kLlmRec implementation (enum value reserved only)
• ValidPathFilter / rec decode constraint integration for the runtime worker (if needed)
• OneRec model implementation + ATB graph building/integration
• Pure NPU inference path

Roadmap checklist

• Rec tokenizer feat: add generative recommendation tokenizer. #317
• Rec decoder constrain feat: add constrained decoding for generative recommendation #480
• Rec proto and service feat: add rec proto,serivce and utils for rec framework[2/6]. #440
• Rec engine master feat: add rec scheduler master and engine for rec framework[3/8]. #557
• Scheduler feat: add rec scheduler master and engine for rec framework[3/8]. #557
• RecType plumbing + rec batch input builder layering feat: add rec_type and onerec batch input builder for rec framework[4/8]. #565
• Rec worker feat: add onerec worker impl for rec framework[5/8]. #567
• request+sampler
• OneRec model + ATB graph integration
• Pure NPU inference

[DragonFive]

I will rebase this pr after #565 and #557 merged.