Preserve funded BEEF in signed actions

[rohenaz]

Summary

• retain the funded BEEF version, transaction entry forms, and BUMPs when appending the signed subject
• validate the completed Atomic BEEF graph up front, rejecting unrelated, incomplete, duplicate, or conflicting proof graphs
• cover proven BUMPs, unproven ancestry, graph failures, standalone subjects, references, and sendWith propagation

Verification

• swift test --filter SignedActionTests (11 tests)
• swift test (250 tests, 10 expected live skips)