feat: Add opt-in scenario-level parallelism (ParallelizationScope.Scenario)

[sourrinn]

Summary

Adds scenario-level parallel execution as a non-breaking, opt-in configuration. When "parallelizationScope": "Scenario" is set in reqnroll.json, scenarios within the same feature run concurrently on independent threads.

Motivation

Closes #114. Feature-level parallelism leaves threads idle when one feature has many more scenarios than others. Scenario-level parallelism achieves linear speedup proportional to total scenario count, not feature count.

Design

Runtime

• Configuration: New ParallelizationScope enum (Feature default, Scenario opt-in) with full JSON schema support
• FeatureLifecycleManager: Reference-counted BeforeFeature/AfterFeature hooks — fire exactly once per feature using Interlocked.CompareExchange + SemaphoreSlim for async-safe mutual exclusion
• ReadOnlyFeatureContextProxy: Copy-on-write isolation — scenario writes go to a thread-local ConcurrentDictionary, shared context remains read-only after BeforeFeature completes
• TestExecutionEngine: Branches on ParallelizationScope in OnFeatureStartAsync/OnFeatureEndAsync; delegates lifecycle to FeatureLifecycleManager in scenario mode
• TestRunnerManager: Work-stealing pool (no feature affinity) in scenario mode; EndFeatureForAvailableTestRunnersAsync is a no-op since lifecycle is per-scenario

Generator

• In scenario mode, class-level fixture setup/teardown bodies are empty
• Each test's cleanup calls OnFeatureEndAsync before releasing the runner
• ParallelizationScope stored in CustomData for providers to inspect

Framework Plugins

Framework	Scenario Mode Behavior
NUnit	Adds [Parallelizable(ParallelScope.Self)], skips [OneTimeSetUp]/[OneTimeTearDown]
xUnit2	Skips IClassFixture/FixtureData pattern
xUnit3	Skips IClassFixture/IAsyncLifetime fixture
MSTest	Skips [ClassInitialize]/[ClassCleanup]
TUnit	Skips [Before(Class)]/[After(Class)] (already supports method-level parallelism)

Configuration

{
  "runtime": {
    "parallelizationScope": "Scenario"
  }
}

Default remains Feature — zero behavior change for existing users.

Thread-Safety Guarantees

1. BeforeFeature fires exactly once — Interlocked.CompareExchange CAS
2. AfterFeature fires only when all scenarios complete — atomic ref-count reaching zero
3. No FeatureContext corruption — ReadOnlyFeatureContextProxy with ConcurrentDictionary overlay
4. No deadlocks — SemaphoreSlim (async-compatible), no nested lock acquisition
5. Error propagation — ExceptionDispatchInfo relays BeforeFeature failures to all concurrent scenarios
6. Static accessors protected — existing DisableSingletonInstance() throws on FeatureContext.Current in parallel mode

Testing

• 2458 existing tests pass (RuntimeTests: 2136, GeneratorTests: 214, PluginTests: 108) — 0 failures
• 9 new stress tests covering:
  • 10-way concurrent initialization (exactly-once validation)
  • Error propagation from failed BeforeFeature to all callers
  • Ref-count finalization (AfterFeature only at zero)
  • Idempotent finalization under concurrent calls
  • Sequential acquire/release lifecycle
  • 20 concurrent scenarios on same feature
  • 10 features x 10 scenarios (100 total) in parallel
  • 50-thread rapid acquire/release
  • Multiple independent feature lifecycles
• Repeated 10x with zero flaky failures

Breaking Changes

None. The default ParallelizationScope.Feature preserves all existing behavior. The constructor parameter is optional with a default value.