feat: add createExternalStore utility and ClockProvider (v4 backport)

[gcutrini]

ref: https://app.clickup.com/t/86b8e70g2

Backport of #195 to the v4.x maintenance branch so event-site (currently on uicore 4.2.29) can adopt ClockProvider without committing to a full MUI v5 -> v6 migration up-front.

What's included

• src/utils/external-store.js — generic createExternalStore factory built on useSyncExternalStore
• src/components/clock-context.js — ClockProvider, useClock, useClockSelector (wraps the existing Clock)
• New webpack entries: components/clock-context, utils/external-store
• New exports from src/components/index.js
• New dep: use-sync-external-store@^1.6.0

Tests

24 new tests (12 for the store factory, 8 for the clock context, 4 misc). Pinned @testing-library/react@^11 for React 16 compatibility (the v5 PR uses v12+). Full suite: 123/123 pass.

Notes

• React 16 currently; per the readme note, the shim import can be replaced with the native react one when the v5 line moves to React 18.
• Functionally identical to feat: add createExternalStore utility and ClockProvider #195. The only deltas are the testing-library version pin and the package.json/yarn.lock layout matching v4.

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