Head & Neck Cancer Therapeutics Market Size & Forecasts 2026-2035, By Segments (Route of Administration, Distribution Channel, Therapy Type), Growth Opportunities, Innovation Landscape, Regulatory Shifts, Strategic Regional Insights (U.S., Japan, China, South Korea, UK, Germany, France), and Competitive Dynamics (Pfizer, Roche, Bristol-Myers Squibb, Merck, Novartis)