feat(fallback)!: handle rejections by default

[thomhurst]

Closes #388

Summary

• give fallback a dedicated default outcome judge that handles ExecutionRejectedException
• preserve retry, circuit-breaker, and hedge rejection defaults
• retain fallback ordering validation when its broader default covers an outer default clause
• document the Polly migration behavior and fallback option contract

Validation

• dotnet build Kevlar.slnx -c Release
• dotnet run --project tests/Kevlar.Tests -c Release --no-build --framework net8.0 -- --timeout 5m
• dotnet run --project tests/Kevlar.Tests -c Release --no-build --framework net10.0 -- --timeout 5m
• pwsh scripts/Verify-ApiDocs.ps1
• pwsh scripts/Verify-Docs.ps1

[coderabbitai]

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[chatgpt-codex-connector]

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