Running Median Monitor

A factory line streams sensor readings all shift, and quality control wants the median reading on demand — averages hide outliers; the median doesn't flinch.

Operations format. Your function receives one argument ops: a list of operations, each itself a list — ["add", x] records a reading, ["median"] asks for the current median. Return a list with one entry per operation: None for each add, and the median for each median query. With an odd count of readings the median is the middle value; with an even count it is the average of the two middle values (may be fractional). Every median call has at least one reading recorded.