refactor: establish Storage folder feature - #334

Tracking issue: #342

Stack context

Part 24 of the Option 5 folder-feature stack.

• Previous: refactor: establish Canvas folder feature #333
• Next: the Workers folder layer

Review this PR as the incremental diff against #333.

Summary

• add the target src/storage folder on the unqualified internal Storage feature
• consolidate Web Storage, Storage Manager, IndexedDB, Cache, and CacheStorage bindings
• add the supported WebAPI.Storage -> WebAPI.Fetch + Storage feature
• preserve all existing flat storage/database/cache module paths

Boundary note

Cache and CacheStorage move out of the old ServiceWorker/WebWorkers folders because they are available outside workers and their Request/Response signatures explain Storage's Fetch dependency. The remaining worker and service-worker interfaces move together in the next PR.

Temporary state

• WebWorkersTypes currently mixes cache and worker support types, so it moves with Storage for this layer; Workers consumes it through WebAPI.Storage, and final helper cleanup can split it after the graph is complete
• the old WebAPI.ServiceWorker and WebAPI.WebWorkers source features are transitional until the immediately following Workers PR
• Window and DomGlobal still reference the cache types and are resolved by the later Window integration
• final feature-graph guardrails and cleanup remain deferred until the stack is complete

Review focus

• Storage membership, especially Cache and CacheStorage
• the Fetch dependency closure
• the temporary mixed support-module ownership

Verification

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