refactor: establish CSSOM folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 15 of the Option 5 folder-feature stack.

• Previous: refactor: establish Geometry folder feature #324
• Next: the DOMNodes folder layer

Review this PR as the incremental diff against #324.

Summary

• add the target src/cssom folder on the unqualified internal CSSOM feature
• move stylesheet, CSS declaration/value, media-query, and style-property-map interfaces out of the legacy DOM folder
• add the supported WebAPI.CSSOM feature
• preserve all nine flat public interface module paths and binding signatures

Dependency direction

The proposal had CSSOM imply DOMNodes. The compiler graph shows the opposite direction:

• CSSOM itself needs only the Event foundation, through MediaQueryList
• DOMTree and Element reference CSSOM-owned types
• making CSSOM imply DOMNodes would therefore create a cycle

This layer defines WebAPI.CSSOM -> WebAPI.Event + CSSOM. The DOMNodes layer will imply CSSOM.

Temporary state

• several CSSOM modules still expose shared records rooted in earlier compatibility types; final type ownership cleanup remains deferred
• this PR introduces no file exclusions or module slices

Review focus

• the compiler-derived acyclic dependency direction
• complete CSSOM folder membership
• unchanged flat public modules

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,CSSOM
• 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. 👍

Reviewed commit: cc0bb1f0d2

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