docs(physics): rustdoc + CHANGELOG to contract statements; α-sensitive katz arm

[tsondru]

Terse-prose plan, step 1 for catgraph-physics (+ CHANGELOG).

Production diff: 0 lines.

• Rustdoc runs > 20 lines in 9 files reduced to contract statements (−293/+57): evolution_graph.rs header, gauge.rs (module, GaugeGroup, HypergraphRewriteGroup, plaquette_action, HypergraphLattice), ollivier_ricci.rs (from_branchial, all_pairs_bfs), branchial_spectrum::spectral_clustering, rewrite_rule, rewrite_span, hypergraph, hyperedge, run_multiway_bfs.
• CHANGELOG.md 442 → 151 lines, one line per change; review restored two dropped facts (parallel default-on in 0.2.2; 0.3.0 public types).
• katz_cycle_diverges_at_large_alpha_converges_at_small refixtured to root→a→b, b→a so the numeric arm moves with α (0.25 and 0.5, tol = 1e-13); α clamped to 0.1 → RED on the 0.25 arm, clamped to 0.25 → RED on the 0.5 arm; reversed export edge → RED; edge weight 2.0 → RED. Null: β is unobservable through L2 normalisation.

Gates: physics tests + clippy on both feature lanes, RUSTDOCFLAGS=-D warnings cargo doc on both lanes, fmt, check_version_refs.py.

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