refactor: establish Crypto folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 7 of the Option 5 folder-feature stack.

• Previous: refactor: establish ViewTransitions folder feature #316
• Next: another dependency-clean domain folder

Review this PR as the incremental diff against #316.

Summary

• rename src/WebCrypto to the target src/crypto folder
• change the folder to the unqualified internal Crypto feature
• add the supported WebAPI.Crypto feature with its WebAPI.DOM dependency
• preserve the flat WebAPI.Crypto and WebAPI.SubtleCrypto modules

The internal WebCryptoTypes helper remains private inside the complete folder.

Temporary state

• this PR does not introduce compatibility aliases, file exclusions, or module slices; the Crypto folder and feature ownership are final
• the typed-array helper it uses remains in core until the final ownership cleanup determines whether that broadly shared type should stay foundational

Review focus

• the WebAPI.Crypto -> WebAPI.DOM + Crypto implication
• unchanged public Crypto APIs and module paths
• complete folder ownership

Verification

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

[jderochervlk]

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