[Web] Add checkout page objects and structured browser coverage

[markmur]

Browser specs currently mix checkout behavior with DOM access, popup setup, and event extraction. Introduce CheckoutHostPage and ShopifyCheckoutPopup so scenarios use named checkout actions and typed state/event reads, and separate synthetic checkout routing from the network guard.

Split the suite into presentation and protocol specs. Bring over the coverage from #418 for registration, URL handling, popup closing, change notifications, window delegation, and terminal errors, adapted to the current lifecycle API. Preserve #855's completion assertions, SDK-derived types, negotiated protocol version, network isolation, and workspace/CI setup.

Stack

• [Web] Add checkout page objects and structured browser coverage #866
• [Web] Add Playwright harness and checkout happy path #855

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	95.16% (374/393)	84.32% (199/236)	97.08% (100/103)

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
Web	npm tarball	89.4 KiB	89.4 KiB	0 B

Web file breakdown

File	Base	Head	Delta
dist/index.js.map	249.1 KiB	249.1 KiB	0 B
dist/index.d.ts	47.9 KiB	47.9 KiB	0 B
dist/custom-elements.json	47.5 KiB	47.5 KiB	0 B
dist/index.js	33.3 KiB	33.3 KiB	0 B
README.md	20.1 KiB	20.1 KiB	0 B
package.json	3.2 KiB	3.2 KiB	0 B
LICENSE	1.1 KiB	1.1 KiB	0 B

Measured from the PR base SHA and PR head SHA. The file breakdown shows uncompressed sizes within each package artifact, so individual files do not sum to the compressed artifact total. This comment reports package artifact sizes only; it is not a final app binary-size report.