refactor: establish UIEvents folder feature - #332

Tracking issue: #342

Stack context

Part 22 of the Option 5 folder-feature stack.

• Previous: refactor: establish Observers folder feature #331
• Next: the Canvas folder layer

Review this PR as the incremental diff against #331.

Summary

• rename src/UIEvents to the target src/ui-events folder
• switch the folder to the unqualified internal UIEvents feature
• add the supported WebAPI.UIEvents -> WebAPI.DOMNodes + WebAPI.File + UIEvents feature
• preserve all existing flat UI event module paths and binding signatures

Temporary state

• the legacy broad WebAPI.DOM source entry remains until its remaining interfaces have been assigned
• final feature-graph guardrails and cleanup remain deferred until the stack is complete

Review focus

• the lowercase folder/internal feature transition
• the DOMNodes/File dependency closure
• preservation of flat public modules

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,DOMPlatform,Animation,CSSOM,Geometry,DOMNodes,File,UIEvents
• npm run build
• npm test
• npm run format:check
• git diff --check