refactor: establish DOMPlatform folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 3 of the Option 5 folder-feature stack.

• Previous: refactor: establish Event folder feature #312
• Next: the DOMNodes folder layer

Review this PR as the incremental diff against #312.

Summary

• add the target src/dom-platform folder on the unqualified internal DOMPlatform feature
• move DOMException, DOMStringList, DOMStringMap, and DOMTokenList into that complete folder
• add the supported WebAPI.DOMPlatform feature with its WebAPI.Event dependency
• move the public interface declarations from the legacy broad DOM source entry without changing their flat module paths

DOMImplementation is intentionally not included: its current signatures embed DOMTree node types, so placing it here would make DOMPlatform depend on DOMNodes while DOMNodes already depends on DOMPlatform.

Temporary state

• the moved modules still reference foundational compatibility types such as DOM.domException, DOM.domStringList, and DOM.domTokenList; the final cleanup layer will make the owning modules authoritative after all consumers have moved
• the legacy broad src/DOM source entry is still tagged WebAPI.DOM, so qualified public-feature selective builds are not yet at their final minimal closure
• DOMStringMap remains an internal supporting module, matching its existing visibility

Review focus

• the dependency-safe DOMPlatform membership
• the WebAPI.DOMPlatform -> WebAPI.Event -> WebAPI.DOM public implication
• preserving flat public module paths and source behavior

Verification

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

[jderochervlk]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

Reviewed commit: ac6324d6db

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