test(physics): refixture cospan contiguity guard with edge_split (#328)

[tsondru]

Closes #328. Audit sweep G13-T1 (physics lane).

What changed

• catgraph-physics/tests/catgraph_bridge.rs — cospan_chain_composability_contiguous_steps refixtured: RewriteRule::edge_split() on Hypergraph::from_edges(vec![vec![0, 1]]), run(.., 3); assert_eq!(cospans.len(), 3); the if cospans.len() >= 2 guard removed; per adjacent pair, the boundary-length equality plus cospans[i].composable(&cospans[i + 1]).is_ok() (use catgraph::category::Composable).
• catgraph-physics/CHANGELOG.md — one bullet under [Unreleased] / Changed.

Prod diff: 0 lines (git diff --stat fb8426b..HEAD — 2 files, both tests/ and CHANGELOG.md).

Premise (measured)

On the pre-fix fixture (A→BB on [[0,1,2]], 3 steps), to_cospan_chain() returned a chain of length 1, so the guard body never ran: PROBE: chain len = 1 — perturbation assert!(cospans.len() >= 2, ...) in a cp -r copy of the base tree, /tmp/audit-reports/G13-T1/probe-guard.log:136.

Falsification (reviewer r1, /tmp/audit-reports/G13-T1/review-r1.md, quoted)

#	Perturbation	Site	Tree	Named test(s) reddened	Observed	Expected
1a	to_cospan_chain closure changed to ignore its window and always rebuild the first pair (cardinality-breaking)	evolution_cospan.rs:46-48	base-commit test file + mutated production	catgraph_bridge.rs: none, 9/9 pass (guard dead: chain len 1 with wolfram_a_to_bb/ternary fixture). Unit: test_multi_step_cospan_chain, test_cospan_chain_composable_via_catgraph, test_compose_multi_step FAILED	boundary "left: 3, right: 2"; composable panic; compose panic "Mismatch in cardinalities"	Reproduces issue #328's measured evidence exactly.
1b	Same mutation, post-fix test file	same	mutant-repeat-first	cospan_chain_composability_contiguous_steps FAILED at catgraph_bridge.rs:99 (length assert_eq!)	left: 3, right: 2	Fix catches what 1a's predecessor did not.
2a	build_cospan_for_pair's right built via child_verts.iter().rev() (cardinality-preserving, label-order-breaking)	evolution_cospan.rs:107	base-commit test file + mutated production	catgraph_bridge.rs: none, 9/9 pass	—	Same dead-guard blindness, independent of mutation kind.
2b	Same mutation, post-fix test file	same	mutant-reverse-right	cospan_chain_composability_contiguous_steps FAILED at catgraph_bridge.rs:106 (composable().is_ok()) — length assert at line 99 passed (3==3)	panic "cospan 0 not composable with cospan 1"	Shows composable() is independently load-bearing, not shadowed by the length assert (1b never reached line 106).

Implementer's falsification (three perturbations: repeat-first-cospan, deterministic_path truncated to 2 nodes, left/right swapped in build_cospan_for_pair; each reddened the refixtured test): /tmp/audit-reports/G13-T1/impl/.

Review rounds

• r1 (54e2a52): 1 important — CHANGELOG bullet absent (applied in 67ba39c); 1 minor — the refixtured test duplicates evolution_cospan.rs unit tests test_multi_step_cospan_chain and test_cospan_chain_composable_via_catgraph assertion for assertion. Kept as the row's "reachable guard" option: it is the public-API (tests/) exercise of to_cospan_chain + Composable; deletion is the row's other option and remains an owner call.
• r2 (67ba39c, delta): 0 findings.

Gates (worktree, --target-dir /tmp/cargo/catgraph-physics; logs under /tmp/audit-reports/G13-T1/)

• cargo nextest run -p catgraph-physics: default 272 passed; --all-features 272 passed; --no-default-features 233 passed (impl/gate-01..03).
• cargo clippy -p catgraph-physics --all-targets -- -D warnings on the same three lanes: clean (impl/gate-04..06).
• RUSTDOCFLAGS="-D warnings" cargo doc -p catgraph-physics --no-deps: clean (impl/gate-07).
• cargo fmt --all --check: clean (impl/gate-08).
• cargo test --workspace -- --nocapture → workspace-tests.log: 147 test result: ok lines, 0 test result: FAILED lines; scripts/check_measured_claims.py workspace-tests.log: exit 0.

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