fix(preact): reuse the parent context by default in HydrationProvider, as React does

[dangreen]

Why

HydrationProvider in @nano_kit/react defaults reuse to true since d833400, where nested hydration boundaries were introduced: a nested provider pushes its dehydrated snapshot into the shared hydrator and renders its children in the parent context. The Preact port never got that change and left reuse undefined, so the same tree created an isolated child context in Preact. The docs mirrored the split: React said "true by default", Preact said "set to true if you want the hydration context to be shared".

What

• packages/preact/src/hydration.tsx: reuse = true, and the prop JSDoc states the default.
• preact.mdx: the reuse prop line now matches the React page.
• Tests in hydration.spec.tsx compare useInjectionContext() inside and outside a nested provider: should reuse the parent hydration context by default (fails on main) and should create a child context when reuse is disabled.

Nothing in the repo nests HydrationProvider in Preact without an explicit reuse: preact-ssr and preact-router do not use it, and the Preact examples go through StaticHydrationProvider.

Checks

• oxlint, tsc --noEmit, vitest run (11 tests) and size-limit in packages/preact pass; the size limits are unchanged (666 B gzip of 700 B, 574 B brotli of 600 B).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.29%. Comparing base (bae248e) to head (2a9586b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   83.33%   83.29%   -0.04%     
==========================================
  Files          98       98              
  Lines        2556     2556              
  Branches      551      551              
==========================================
- Hits         2130     2129       -1     
  Misses        314      314              
- Partials      112      113       +1     

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