Conclude#follow accepts _id and makes every generated fact violate the ID rule

[gemshrine]

Fbe::Conclude#follow allows _id, even though every new fact in the FBE factbase receives its own _id automatically. Following a source fact's _id then appends that old identifier to the new fact's already assigned identifier, violating the one-ID rule and aborting the transaction.

Steps to reproduce:

1. Put a fact with an _id and a what value in the factbase.
2. Run Fbe.conclude with a matching on query, follow '_id', and a draw block that creates a normal result.

Actual result: follow accepts _id, then fill copies it with fact.public_send(:_id=, v). Fbe.fb assigns a new _id when the result fact is inserted, so the copy makes that fact contain two _id values. The configured unique-ID rule raises ArgumentError and the conclusion rolls back instead of producing the result.

Expected result: reject _id in follow with an actionable Fbe::Error, like the existing checks for what and details, or explicitly exclude system identity fields from copying.

Technical evidence: lib/fbe/conclude.rb rejects only what and details in follow, and fill copies every listed property. lib/fbe/fb.rb assigns a fresh _id on insertion, while the FBE Factbase is wrapped in Factbase::Rules with uid: '_id', which rejects a fact containing more than one _id.