data: add Mixedbread startup credits

[xaviderds3-tacna]

What changed

Added one data-only vendor record for mixedbread with one source-backed program and one offer. Closes #730.

Sources

• https://www.mixedbread.com/pricing

Certification

• I changed only allowed repository content and did not mix vendor YAML with other paths.
• Public sources substantiate every submitted factual value; Sourcey-written prose is a neutral summary.
• I did not author verification, provenance, freshness, or signature claims.
• My commit includes a Signed-off-by line.

AI-assisted contribution, reviewed by the operator and validated with the pinned Catalog Verifier.