fix(engine): fault empty async throws

[thomhurst]

Summary

• return a faulted ValueTask when an empty shield delegate throws synchronously
• cover typed, state-passing, Task-returning, outcome, cancellation, sync/async completion, and Retry(0) parity
• repair the post-merge invocation-semantics test to use the renamed FallbackTo API

Performance

OverheadBenchmarks.Kevlar_Empty, ShortRun, .NET 10.0.11, same machine:

	Mean	Allocated
main	7.909 ns	0 B
this branch	14.27 ns	0 B

The exception boundary adds 6.36 ns to the empty happy path while preserving its zero-allocation contract. Inline EH was the fastest correct implementation measured; helper and async-state-machine variants measured 15.68 ns and 22.34 ns respectively.

Test plan

• dotnet build Kevlar.slnx -c Release (0 warnings)
• Kevlar.Tests (835 passed)
• Kevlar.IntegrationTests (123 passed)
• Kevlar.AllocationTests (3 passed)
• Kevlar.NetStandard.Tests (10 passed)
• BenchmarkDotNet Dry validation and before/after ShortRun

Closes #228

[coderabbitai]

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