feat: add portable commerce block set

[superposition]

What this adds

Eight accessible copy-to-own commerce blocks: product card, catalog filters, cart line item, order totals, shipping address, payment routes, payment status, and order receipt.

Host contract

The consuming application owns routes, hidden identifiers, CSRF, authentication, catalog data, money calculations, inventory, wallet integration, settlement, shipping, tax, persistence, and policy. Blocks preserve semantic HTML and native GET/POST behavior only.

Evidence

• 65 Coatcheck story/scene combinations across mobile, tablet, desktop, light, and dark
• 9 project-specific semantic browser checks
• 74 total browser/Axe checks passed
• keyboard, console, overflow, serious/critical Axe gates passed
• cargo fmt, test, clippy, package isolation, and release smoke passed
• selected non-baseline review PNGs committed under reviews/commerce

Release boundary

The crate is bumped to 0.2.0. Coatcheck, Playwright, workbench source, and review images remain outside the published package.