refactor: establish Observers folder feature - #331

Tracking issue: #342

Stack context

Part 21 of the Option 5 folder-feature stack.

• Previous: refactor: establish SVG folder feature #330
• Next: the UIEvents folder layer

Review this PR as the incremental diff against #330.

Summary

• add the target src/observers folder on the unqualified internal Observers feature
• consolidate Intersection Observer, Mutation Observer, and Resize Observer bindings
• add the supported WebAPI.Observers -> WebAPI.DOMNodes + WebAPI.Geometry + Observers feature
• preserve the four existing flat observer interface modules
• localize the instanceof Document check in IntersectionObserverRoot so the observer feature does not depend on the broad legacy Document module

Dependency note

IntersectionObserverRoot previously imported Document only for its runtime type check. Its public type already uses foundational DOM.document; keeping the check local removes an otherwise unnecessary dependency on the still-broad Document interface without changing the decoded result.

Temporary state

• Document remains in the legacy DOM folder until the later Window/document integration layer
• 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

• observer domain membership
• the DOMNodes/Geometry dependency closure
• the localized Document runtime check

Verification

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