The write_envelope tool was silently accepting YAML without a 'facts:'
top-level key. serde would ignore unknown fields and default the facts
HashMap to empty, causing the predicate pipeline to always see no facts.
Now validates that envelope.facts is non-empty after deserialization,
returning a clear error with an example of the correct format.
Adds 6 tests covering valid/invalid/boundary deserialization cases.
b045d0c5e9