data: add Strix for Startups

[zmx2013]

What changed

Adds Strix and its current Strix for Startups program with one startup-specific offer.

The record captures:

• 50% off the Strix Pro plan
• Six-month program duration
• Eligibility for early-stage startups from pre-seed through Series A
• Work-email requirement and manual application review
• The public first-party startup application route
• Supporting Pro-plan context from Strix's first-party pricing page

The change is data-only and adds exactly one file:

vendors/st/strix.yaml

Closes #770

Sources

• https://www.strix.ai/startups
• https://www.strix.ai/pricing

Certification

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