feat(useCountdown): add useCountdown hook

[JeongHwan-dev]

Overview

Add useCountdown hook: useCountdown is a React hook that manages a countdown timer with precise control. It provides functions to start, pause, resume, and reset the countdown. The hook tracks the current count value and supports both incrementing and decrementing modes.

Checklist

• Did you write the test code?
• Have you run yarn run fix to format and lint the code and docs?
• Have you run yarn run test:coverage to make sure there is no uncovered line?
• Did you write the JSDoc?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c76f1fe) to head (3eab881).
Report is 6 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           33        34    +1     
  Lines          839       884   +45     
  Branches       254       265   +11     
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+ Hits           839       884   +45     

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[seungrodotlee]

How about changing it to a more intuitive name? (e.g., useTimer)

[kimyouknow]

@JeongHwan-dev Thank you for the excellent implementation — the code quality is impressive (252 lines of tests, clean TypeScript, great use of existing hooks).

Before merging, we'd like to discuss one scope question:

useCountdown is a composition of existing primitives (useCounter + useBooleanState + useInterval). We're currently discussing whether react-simplikit should focus on primitive building blocks that users compose themselves, or also include composed convenience hooks.

If we include useCountdown, similar composed hooks (useTimer, useStopwatch, etc.) would follow the same precedent.

We'd like to align with @zztnrudzz13 on this before proceeding. Will follow up soon!

[kimyouknow]

@JeongHwan-dev Following up — after team discussion, we've decided that composed hooks like useCountdown fall outside our library's scope.

react-simplikit focuses on providing primitive building blocks only.

useCountdown composes existing primitives (useCounter + useBooleanState+ useInterval), and including it would set a precedent for similar composed hooks (useTimer, useStopwatch, etc.).

Same rationale as #338, #343 — we're focusing on primitive building blocks only.

Users can compose these primitives to build their own countdown logic. Thank you for the great implementation!