Extract targetGraphComparer into separate module

[yushan8]

• Extract targetGraphComparer into controller/comparer.go — moves the graph comparison logic (target diffing, BFS distance computation, ID remapping, seed classification, metadata chunking) out of getchangedtargets.go into a focused, self-contained type
• GetChangedTargets calls newTargetGraphComparer(...).Compare(...) directly instead of going through a compareTargetGraphs wrapper method on the controller
• Helper functions that only serve the comparison algorithm (transposeOptimizedTarget, computeDistances, changedDepStatus, hasChangedSourceFileDep, attributesChanged, getTargetsAndMetadata, buildNameIndex, detectSourceFileID, canonicalTargetName, validateTargetNames) move to comparer.go
• Tests that exercised compareTargetGraphs now use newTestComparer().Compare(...) directly, removing the need for newTestController in those tests