AffectedGraphAnalyzer

Computes the affected graph for a diff: which modules a set of changed files touches, and the downstream blast radius that a build must therefore rebuild and retest.

A change to module X forces everything that depends on X to rebuild, so the affected set is the changed modules plus every production dependent reachable from them (reverse reachability over non-test edges). This turns Aalekh's static blast-radius metric into a per-pull-request signal. Pure - the changed file list is supplied by the caller (the plugin reads it from git diff).