refactor: consolidate Messaging folder feature

[jderochervlk]

Stack context

Part 12 of the Option 5 folder-feature stack.

• Previous: refactor: consolidate Device folder feature #321
• Next: the Storage and Workers folder layers

Review this PR as the incremental diff against #321.

Summary

• add the target src/messaging folder on the unqualified internal Messaging feature
• merge channel messaging, notifications, push, and WebSockets into that complete folder
• replace four qualified source features with the supported WebAPI.Messaging bundle
• define WebAPI.Messaging -> WebAPI.Event + WebAPI.File + Messaging
• preserve all existing flat public interface modules

Temporary state

• this PR introduces no exclusions, compatibility aliases, or module slices; the Messaging folder membership and public implication are final
• its private domain *Types helpers remain inside the correct folder pending final cleanup
• Service Worker interfaces are intentionally not included here; they move into Workers after the Storage dependency is established

Review focus

• the coarse Messaging bundle membership
• removal of the former ChannelMessaging, Notification, Push, and WebSockets consumer features
• preservation of public module paths and signatures

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,File,Messaging
• npm run build
• npm test
• npm run format:check
• git diff --check