Builder signs receipts with empty fields and out-of-range scores

[royalpinto007]

Problem

The builder only checks None: empty query/answer, empty source id/content, and score=999.0 all finalize and verify as valid receipts.

Where

src/answerproof/builder.py (_validate checks is None only, no emptiness or [0, 1] range checks).

Acceptance

Add a failing test first finalizing with "" query/answer/source fields or an out-of-range score, expecting ValueError. Then fix.