Add protocol-agnostic checkout events for web

[markmur]

Adds typed start, update, complete, error, and close events for observing the web checkout lifecycle. Consumers receive Kit-owned snapshots and error payloads through standard DOM event listeners, with protocol handling kept internal.

• Introduces a Kit-owned Checkout snapshot that removes only top-level ucp, reuses nested domain types, and preserves optional fields and inline extensions.
• Combines line-item, message, total, and fulfillment notifications into structurally deduplicated updates, with fresh snapshot state for each presentation.
• Exposes stable error codes and diagnostic messages while preserving terminal protocol-error handling: the component emits error, closes the session, and emits close, regardless of message severity.
• Documents the public lifecycle API and updates the sample, declarations, and custom-element metadata.

Window-open requests continue to use the existing internal HTTPS opening behavior.

Stack

• Add protocol-agnostic checkout events for web #766
• Add protocol-agnostic checkout events for Android #767
• Protocol-agnostic checkout events for Swift #748
• Namespace Swift protocol payloads #777

[github-actions]

Package Size

Platform	Artifact	Base	Head	Delta
Web	npm tarball	80.1 KiB	80.5 KiB	+404 B

Web file breakdown

File	Base	Head	Delta
dist/index.js.map	217.7 KiB	221.2 KiB	+3.5 KiB
dist/custom-elements.json	51.4 KiB	48.0 KiB	-3.5 KiB
dist/index.d.ts	45.9 KiB	35.0 KiB	-10.9 KiB
dist/index.js	39.5 KiB	40.3 KiB	+761 B
README.md	20.0 KiB	20.1 KiB	+51 B
package.json	2.9 KiB	2.9 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.

[bitrise]

Install this build

Open Tophat, select your target device, then click Install. Links open on the Mac running Tophat.

SDK	Install
Kotlin	Install with Tophat

Checkout Kit E2E results

Status	Suite	Target	Platform	OS version tag	Device
✅	kotlin-android	kotlin	android	latest	Google Pixel 9 Android 17.0

[github-actions]

Web — Coverage Report

Lines	Statements	Branches	Functions
	95.17% (375/394)	84.32% (199/236)	97.08% (100/103)