Improve reusable TestCase and Swift Testing integration

[kudit]

What changed

This draft PR keeps the work in two reviewable commits:

1. Consolidate TestCase execution and preserve diagnostics

   • Routes execute() and the live-UI run() path through one lifecycle implementation.
   • Preserves async setup/test/teardown behavior without attempting to await inside defer.
   • Adds .parallel and .serialized execution modes for tests that mutate process-global state.
   • Introduces TestFailure, retaining file, function, line, column, and detailed expectation values.
   • Logs expectation failures at .ERROR before throwing so command-line runs retain useful diagnostics.

2. Add reusable Swift Testing module adapter

   • Adds a Compatibility Testing Library product in the same package and .swiftpm project.
   • Adds ModuleTestEntry, one Swift Testing argument per Compatibility TestCase.
   • Uses CustomTestStringConvertible for readable Xcode names such as Device › Battery Tests › Reports current battery state.
   • Uses CustomTestArgumentEncodable for stable argument identity and selective reruns.
   • Lets consuming modules reduce their adapter to ModuleTestEntry.entries(including:) and entry.execute().

Design decisions

• TestCase remains @MainActor because it owns observable UI progress and a mutable test closure. A sendable execution snapshot keeps detached execution safe and prevents the UI model from crossing actors.
• main {} remains distinct from await MainActor.run {}: the former schedules fire-and-forget work, while the latter awaits completion and can return a value.
• The broader main/background/sleep/WASM overload consolidation belongs to follow-up PR Consolidate threading APIs and add WebAssembly async fallbacks #3.

Open draft review items

Before this PR is marked ready for review:

• make .serialized execution truly exclusive against both serialized and parallel reusable tests;
• mark the debug-configuration test serialized;
• restore debug configuration with test-local synchronous defer rather than automatically resetting all Compatibility settings around every test;
• compile and run the package tests in Xcode and SwiftPM across the supported platform matrix.

Validation so far

• New Swift files were syntax-parsed with Swift 6.2.1.
• Full package compilation and platform validation remain required before merge.

Dependent follow-up

After this API is finalized and released, create a Device pull request that imports CompatibilityTesting and replaces its duplicated Entry/module lookup adapter with ModuleTestEntry.