refactor: establish Geometry folder feature - #324

Tracking issue: #342

Stack context

Part 14 of the Option 5 folder-feature stack.

• Previous: refactor: establish Animation folder feature #323
• Next: another dependency-clean DOM-adjacent folder

Review this PR as the incremental diff against #323.

Summary

• add the target src/geometry folder on the unqualified internal Geometry feature
• move the DOM matrix, point, and rectangle interface families out of the legacy broad DOM folder
• add the supported WebAPI.Geometry feature with its WebAPI.DOM dependency
• preserve all seven flat public module paths and binding signatures

Temporary state

• the modules still use foundational DOM.* geometry records introduced by the earlier compatibility cleanup; the final cleanup layer will make the owning Geometry modules authoritative after all consumers move
• this PR introduces no exclusions or module slices
• DOMNodes, SVG, Observers, and Canvas dependencies on Geometry are added in their owning layers

Review focus

• complete Geometry folder membership
• the WebAPI.Geometry -> WebAPI.DOM + Geometry implication
• unchanged public interfaces

Verification

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