fix(cdp): serialize integral box-model coords as JSON integers (#576)

DOM.getBoxModel emitted every coordinate through json!(f64), so an integral
value like 256.0 serialized as "256.0". Strict CDP clients (Hermes Agent)
deserialize box-model coordinates as i64 and reject the float, breaking every
click-by-coordinate flow. Chrome only widens genuinely fractional coordinates,
so mirror that: coord_value() collapses an exactly-integral, i64-representable
double to a JSON integer and leaves fractional / NaN / inf values untouched.

Adds a unit test covering integral, fractional and mixed-quad serialization.