Find feature flags that are safe to remove

Your codebase carries hundreds of feature flags, many fully rolled out years ago. An agent opens a pull request to remove a flag, but only when every observation of it shows the same settled value for long enough.

Write select_removable_flags(observations, min_flat_days): the rule your agent follows to pick the flags it may remove.

• observations (list of str): one per flag, environment and segment, written name,environment,segment,serving_percent,flat_days, for example "checkout_v2,prod,default,100,180". serving_percent is the share of traffic on the new path, and flat_days is how long it has stayed there. Both are whole numbers.
• min_flat_days (int): how long every observation must have been flat.
• Return the removable flag names, each once, sorted in ascending order.

A flag is removable only if all 3 hold:

1. Every observation of it has the same serving_percent, across all environments and segments.
2. That shared value is 0 or 100.
3. Every observation of it has flat_days of at least min_flat_days.