No library changes — SPM consumers are unaffected.
The test suite (37 suites, 179 tests) is migrated from XCTest to Swift Testing: @Test functions with #expect/#require, value-type suites, per-test seeded RNGs, and parallel execution by default. Availability-gated suites (Zip) carry @available per test function, specific-error expectations use #expect(throws: SomeError.case), and the Sudoku validity helper threads sourceLocation so failures point at the calling test.
Validated on Swift 6.0.2 (macOS + Linux), the current Xcode toolchain, and the latest Swift Linux image.