fix(dns): report TCP fallback as inspection metadata

[ryanfowler]

Summary

• render successful UDP-to-TCP fallback as structured inspection metadata
• expose fallback record types at extra verbosity without stderr warnings
• preserve fallback metadata when TCP retry fails or system resolver failover continues
• add regression tests and documentation

Validation

• go test -v -parallel 8 ./...
• go build .
• CGO_ENABLED=0 go build
• staticcheck ./...
• go mod tidy && go mod verify