refactor: consolidate Messaging folder feature

[jderochervlk]

Tracking issue: #342

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

[jderochervlk]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep the moved messaging modules in the API docs

When Astro generates the API reference, docs/utils.js:85-92 derives each source directory from featureSpecs, which still lists ChannelMessaging, Notification, Push, and WebSockets (scripts/unmonorepo/feature-spec.mjs:77,95,99,113). After moving those files into src/messaging, none of their expected *Types.res paths exist, so the filter silently drops all four API groups and their public modules—including MessagePort, Notification, the Push modules, and WebSocket—from the generated routes and index. Update the documentation metadata/path handling as part of this move.

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