refactor: establish WebAudio folder feature - #329

Tracking issue: #342

Stack context

Part 19 of the Option 5 folder-feature stack.

• Previous: refactor: establish Media folder feature #328
• Next: the SVG folder layer

Review this PR as the incremental diff against #328.

Summary

• rename src/WebAudio to the target src/web-audio folder
• switch the folder to the unqualified internal WebAudio feature
• add the supported WebAPI.WebAudio -> WebAPI.Media + WebAPI.Messaging + WebAudio feature
• preserve all existing flat WebAudio module paths and binding signatures

Dependency note

The Option 5 proposal listed only Media as a public dependency. The current bindings also expose MessagePort from AudioWorkletNode.port, so the compiler-proven closure additionally requires WebAPI.Messaging. This PR records the real public signature dependency at the domain level.

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 additional Messaging dependency
• preservation of flat public module names

Verification

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