data: add HeyGen startup offer

[albertstayhome]

Adds HeyGen as a new Sourcey entity with its startup program offer, sourced from the official HeyGen startup page. Refs #839.

Checklist:

• data-only change under entities;
• one new entity YAML;
• one current offer;
• signed-off commit.

[auscaster]

Thank you for this contribution, and apologies for the slow review.

I checked this record against its cited sources by hand. It needs a correction before it can be admitted:

cited source 404s: https://www.heygen.com/startups.

Every published figure has to be supported by a canonical source: a page on the vendor's own domain, or on the domain of an entity named in the offer's roles. Please correct the record or cite a better public source on this same pull request, and I will re-review.

_{Reviewed by hand against CONTRIBUTING.md and the canonical-source rule, 28 Aug 2026.}