feat(kida): add latest for the value of the source that changed last

[dangreen]

What

latest(...sources) returns a signal carrying the value of whichever source changed most recently.

const $searchQuery = signal('')
const $searchParam = searchParam($searchParams, 'q', v => v ?? '')
const $search = latest($searchQuery, $searchParam)

Typing wins over the URL, navigation wins over what was typed. It is the missing half of previous from @nano_kit/store: that one looks back through a single source, this one picks the freshest across several. It lives in kida, so @nano_kit/store re-exports it too.

Contract

• The last source wins on the first read, whatever its value, undefined included.
• The last source also wins whenever several sources changed since the previous read. That covers writes inside one batch, sources sharing an upstream, and reads made without a subscription.
• A change is a change of value: a write of an equal value is invisible, and so is a → b → a inside one batch.
• The winner is kept, not recomputed. Losing the last subscriber purges the deps and leaves the node dirty, so the next read recomputes with nothing changed; a util that re-picked a winner there would flip its answer on every remount.
• Every source is read on every run. An early exit on the first change would drop the subscriptions of the sources after it.

Tests

Seven cases in packages/kida/src/utils.spec.ts pin every rule above. They were checked against wrong implementations rather than only against a green run:

mutation	caught by
drop the first-run branch	should return the last source value on the first read when it is undefined
break after the first change	five of seven, subscriptions are lost
re-pick the winner instead of keeping it	should keep the value when nothing changed since the last read

Size

latest costs 60 B gzip and 52 B brotli in kida's All publics: 4989 → 5049 and 4604 → 4656. Both All publics entries are re-pinned in kida and in store, which mirrors them through the re-export. Signal and Popular set are untouched, the util is not in those sets.

Verification

kida unit tests green (87), oxlint and tsc --noEmit clean, test:size run sequentially across all 13 packages with no exceedances.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.75%. Comparing base (b911d4c) to head (8dafa2a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #231   +/-   ##
=======================================
  Coverage   82.75%   82.75%           
=======================================
  Files          98       98           
  Lines        2465     2465           
  Branches      521      521           
=======================================
  Hits         2040     2040           
  Misses        315      315           
  Partials      110      110           

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