refactor: establish Workers folder feature - #335

Tracking issue: #342

Stack context

Part 25 of the Option 5 folder-feature stack.

• Previous: refactor: establish Storage folder feature #334
• Next: the Navigator folder layer

Review this PR as the incremental diff against #334.

Summary

• add the target src/workers folder on the unqualified internal Workers feature
• consolidate Web Worker and Service Worker interfaces
• add the supported WebAPI.Workers -> WebAPI.Messaging + WebAPI.Storage + Workers feature
• preserve all existing flat worker and service-worker module paths
• remove the transitional old WebAPI.ServiceWorker and WebAPI.WebWorkers source features

Boundary note

Cache and CacheStorage remain in Storage from the previous PR. Workers receives their types through WebAPI.Storage, while WebAPI.Messaging supplies MessagePort, MessageEvent, push, and notification-related dependencies.

Temporary state

• the mixed WebWorkersTypes support module remains in Storage until final helper cleanup; worker interfaces consume it transitively through the declared Storage dependency
• Navigator.serviceWorker remains in the legacy Navigator interface until the next layer
• final feature-graph guardrails and cleanup remain deferred until the stack is complete

Review focus

• Workers domain membership
• the Messaging/Storage dependency closure
• resolution of the previous PR's transitional source features

Verification

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