feat(analyzers): add KEV005 fallback check

[thomhurst]

Summary

• add KEV005 for void fallbacks used by provably result-returning executions
• cover fluent chains, stable locals/aliases, composition, context-aware methods, outcome methods, and Task extensions
• document the diagnostic and track its analyzer/public API release entries

Scope decision

Tier 3 field/property flow is deferred. Existing analyzer data flow proves stable local initializers only; inferring single-assignment members safely needs interprocedural/member-write analysis. Fields, properties, parameters, returned shields, and reassigned locals remain conservative false negatives.

Test plan

• KEV005 focused tests (6/6)
• dotnet run --project tests/Kevlar.Analyzers.Tests -c Release -- --timeout 5m (53/53)
• dotnet build Kevlar.slnx -c Release (0 warnings)
• npm run build (docs/)
• package and run scripts/Verify-DocSnippets.ps1 (97 snippets compiled; documented pipeline executed)

Closes #160

[coderabbitai]

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