SPP1 and head and neck squamous cell carcinoma: Single-cell RNA and ATAC assays in HNSCC converge on a structured view of tumor-associated myeloid biology: SPP1+ macrophages and suppressive MDC/MDSC modules organize immune-excluded niches; cDC1 insufficiency and migratory LAMP3+ DC reprogramming constrain priming; and neutrophil states reinforce chemokine and matrix circuits.