Add GraphCompressorTest: 29 tests covering all 5 compression strategies

GraphCompressor.java had no JUnit coverage (one of the larger untested
classes in the gvisual package - 482 lines, 5 distinct strategies,
nontrivial Jaccard pruning, plus a full CompressionResult value type).

New GraphCompressorTest.java (29 @test methods) exercises:

  * Argument validation: null graph, null attribute fn, null/empty
    seeds, threshold out of [0,1], binSize < 1, k < 1
  * Empty + singleton graphs (degenerate edge case for divisions)
  * Structural equivalence on K_{2,3} (asserts exact node/edge counts,
    grouping, ratio 0.4, edge-reduction 83.3%, aggregated weight = 6)
  * Structurally distinct nodes (path graph) ⇒ no merges
  * Neighborhood similarity at thresholds 0.0, 0.5, 1.0
  * Exact degree binning (star graph) + binSize=2 coarsening
  * Attribute compression: label-based grouping, null bucketing via
    __unassigned__, intra-group edge dropping in the quotient
  * k-hop locality: seed neighborhood capture, already-claimed
    skipping with two seeds, unknown-seed handling
  * compressibilityReport contains every strategy line
  * Result accessors: getSummary, toCSV (sorted nodes), getMembersOf,
    getSupernodeOf, unmodifiable maps
  * Superedge weight aggregation (sum of crossing originals)

All 29 tests green: 'mvn -Dtest=GraphCompressorTest test' → Tests run:
29, Failures: 0, Errors: 0, Skipped: 0.

Author: zalenix-gardener