Improve reusable TestCase and Swift Testing integration

[kudit]

What changed

This PR modernizes Compatibility's reusable test infrastructure and related diagnostics while preserving the existing in-app TestCase model.

Reusable test execution

• Unifies TestCase.execute() and live-UI execution through one lifecycle implementation.
• Preserves async setup/test/teardown behavior and detailed source-aware failures.
• Adds explicit parallel/serialized execution behavior for tests that mutate process-global state.
• Restores mutable debug settings with test-local defer even when expectations throw.

Swift Testing integration

• Adds the Compatibility Testing Library product.
• Adds ModuleTestEntry, adapting each reusable TestCase into a parameterized Swift Testing case with stable identity and readable descriptions.
• Keeps infrastructure-only target tests separate from reusable module tests.
• Updates the Development Xcode test configuration so the shared CompatibilityTest scheme runs both unit and UI test bundles with code coverage enabled.

Debug/source-context cleanup

• Consolidates debug processing around DebugFormatContext and SourceContext.
• Keeps direct #file / #function / #line / #column capture at public call boundaries while forwarding a single SourceContext internally.
• Uses the structured debug formatter as the canonical formatting path while retaining legacy compatibility adapters where appropriate.

Threading portability

• Removes misleading WASM/Embedded fallbacks for main, background, sleep, and delay when Compatibility cannot provide the advertised scheduling/timing semantics.
• Supported threaded runtimes continue to use real actor/task/dispatch behavior.

Package/version cleanup

• Synchronizes the pending release surfaces at v1.18.3.
• Expands contributor guidance and release/test documentation.

Validation before merge

• Run swift test and confirm all reusable module parameter cases pass.
• Run the shared CompatibilityTest Xcode scheme and confirm both CompatibilityTests and CompatibilityUITests are discovered and pass.
• Confirm parameterized test cases are visible/selectively runnable in Xcode after the scheme update.
• Review code coverage for meaningful non-UI gaps.
• Run supported-platform / Swift Package Index validation before tagging v1.18.3.

Follow-up

After release, Device can import CompatibilityTesting and replace its duplicated module-test adapter with ModuleTestEntry.

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