Identify Rowset brand in product structured data

[gregagi]

Summary

• Declare the actual Rowset brand in pricing-page Product JSON-LD, addressing Google's merchant-listing product-identity suggestion without inventing a GTIN or MPN.
• Availability was already fixed in Fix hosted offer availability in structured data #417. Leave shipping and physical merchandise return fields absent for the hosted software subscription; preserve all offer/pricing data.

Verification

• Checked Google's merchant-listing documentation for brand.name.
• git diff --check passed; existing full CI required before merge; live pricing markup verification after deployment.
• SEO-only change: no changelog entry per repo AGENTS.md.