Document breaking change policy

ex: we will not change scoring for performance or accessibility w/o a major version bump, but will for best practices. what about other categories?

Start by locating the current documented breaking-change policy and reviewing how scoring categories are handled. Define which categories require a major version bump and which may change without one, including the unresolved “other categories” question. Done means the policy clearly covers all relevant categories and versioning expectations.

Written by the indexing model from the issue text.