A 5-year-old girl from Delhi is brought to the emergency department with a 4-day history of acute gastroenteritis with severe diarrhea and vomiting. On examination, she is lethargic, with sunken eyes, absent skin turgor, weak radial pulses, and a capillary refill time of 4 seconds. Blood pressure is 78/48 mmHg (< 5th percentile for age). Respiratory rate is 38/min with deep breathing. Laboratory findings: pH 7.18, HCO₃⁻ 12 mEq/L, PaCO₂ 28 mmHg, Na⁺ 155 mEq/L, K⁺ 6.2 mEq/L, Cl⁻ 118 mEq/L, BUN 52 mg/dL, creatinine 1.8 mg/dL. Urine output is < 0.5 mL/kg/hr. What is the primary shock type and the most appropriate fluid for initial resuscitation?