Function buildViolationPlan

Build an ordered fix plan from file-level violations.

• violations are grouped per file (and tagged with the file's layer)
• the import graph is projected onto the violated files: file A depends on violated file B when B is reachable from A without passing another violated file (clean files and barrels in between are walked through)
• import cycles among violated files become one cluster (Tarjan SCC) that has to be fixed together
• clusters are ordered dependencies first; the topological level of a cluster decides its phase. Within a level single files come before cycle clusters, then files are sorted by layer and path.

Pure: the caller supplies the dependency lookup and the layer lookup.