feat(kida): export child to build custom child signals

[dangreen]

Why

The Child Signals section of the store low-level page documents child($parent, key, setValue), but child lived only in packages/kida/src/internals and was not exported from kida or @nano_kit/store, so the documented import did not resolve. The public helpers built on it, atKey and atIndex, are exported and covered by the Advanced page; child is the primitive for custom writers.

What

• packages/kida/src/child.ts re-exports internals/child.js, following lifecycle.ts, and index.ts exports it. @nano_kit/store picks it up through export * from 'kida'.
• Docs: the dynamic key example types the key signal as signal<'a' | 'b'>('a'); child constrains the key to keyof P, so an Accessor<string> is rejected.

Checks

• The three docs snippets were type-checked against the real @nano_kit/store exports, including the expected errors for an untyped key and for writing a readonly child.
• oxlint, tsc --noEmit and vitest run (87 tests) in packages/kida pass; child keeps its own internals/child.spec.ts.
• Sequential build of all packages and size-limit of every package that has it pass with unchanged limits: child was already bundled through atKey and atIndex.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (cb2d9e2) to head (9960193).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #250   +/-   ##
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  Coverage   83.33%   83.33%           
=======================================
  Files          98       98           
  Lines        2556     2556           
  Branches      551      551           
=======================================
  Hits         2130     2130           
  Misses        314      314           
  Partials      112      112           

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