style(fmt): unbreak rust-ci Test job (cargo fmt drift) (#36)

## Summary

`cargo fmt --check` has been red on `main` since 2026-05-22, blocking
the rust-ci **Test** job on every push. Two let-bindings in
`tests/seam_contract_tests.rs` (lines 433 and 490) were wrapped onto two
lines despite fitting on one.

Pure mechanical `cargo fmt` output — no semantic change.

## Test plan

- [x] `cargo fmt --check` exits 0 locally
- [ ] CI on PR: rust-ci `Test` job goes green

## Out of scope

The `Check MSRV (1.85.0)` job is also red on main (eframe pulls in deps
requiring rustc 1.92+) — that needs an MSRV bump or a downgrade
decision, tracked separately.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath