refactor: establish Performance folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 5 of the Option 5 folder-feature stack.

• Previous: refactor: establish URL folder feature #314
• Next: another dependency-clean domain folder

Review this PR as the incremental diff against #314.

Summary

• rename src/Performance to the target src/performance folder
• change the folder from the qualified source feature to the unqualified internal Performance feature
• add the supported WebAPI.Performance feature with its WebAPI.Event dependency
• preserve the flat Performance, PerformanceEntry, and PerformanceMark public modules

The internal PerformanceTypes helper remains private within the complete folder.

Temporary state

• this PR does not introduce file exclusions or module slices; the Performance folder ownership and public implication are final
• the interfaces still use the existing internal PerformanceTypes aggregation; type colocation can be handled in the final cleanup after the folder graph is complete

Review focus

• the WebAPI.Performance -> WebAPI.Event + Performance implication
• unchanged public modules and signatures
• complete folder ownership without cross-domain additions

Verification

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

[jderochervlk]

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[chatgpt-codex-connector]

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