Incremental refresh of an existing codebase survey. With no argument, each module is diffed from its own recorded surveyed_sha to HEAD. With a commit range or PR# argument, that range is used uniformly. Maps changed paths to affected module docs, dispatches narrowly-scoped subagents for only those modules, then bumps each touched module's surveyed_sha. Model-invocable so it can be wired to a post-merge hook later.
