fix(router,ssr): render the httpEquiv meta prop as the http-equiv attribute

[dangreen]

Why

Head descriptor props follow the React naming, and httpEquiv is the only prop whose HTML attribute name is not just its lowercase form. syncHead passed the prop name straight to setAttribute and into the hydration selector, and headDescriptorToHtml only lowercased it, so meta({ httpEquiv: 'refresh', content: '...' }) produced <meta httpequiv="refresh"> both on the client and in server HTML. Browsers ignore that attribute, and on hydration the selector meta[httpEquiv="refresh"] did not match the server tag, so a second meta was appended.

What

• @nano_kit/router exports toHtmlAttribute(prop): httpEquiv becomes http-equiv, everything else is lowercased. syncHead uses it for the hydration selector and setAttribute.
• headDescriptorToHtml in @nano_kit/ssr uses the same helper instead of its own toLowerCase().
• Tests: toHtmlAttribute unit tests, should render httpEquiv as the http-equiv attribute and should hydrate existing http-equiv meta tag in the router meta suite (both fail on main: httpequiv, and a duplicated meta), and the SSR counterpart in headDescriptorToHtml.

Size

The public helper costs bytes in the router bundle; the limit is re-pinned on the 0.05 kB step:

bundle	before	after	limit
All publics (Gzip)	4121 B	4172 B	4.15 kB → 4.2 kB
All publics (Brotli)	3703 B	3748 B	3.75 kB
Minimal set (Gzip)	1543 B	1543 B	1.55 kB
Minimal set (Brotli)	1370 B	1368 B	1.4 kB

Checks

• oxlint, tsc --noEmit and vitest run pass in packages/router (135 tests) and packages/ssr (15 tests); size-limit passes in packages/router.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (ab83437) to head (478f627).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #253   +/-   ##
=======================================
  Coverage   83.33%   83.33%           
=======================================
  Files          98       98           
  Lines        2556     2557    +1     
  Branches      551      552    +1     
=======================================
+ Hits         2130     2131    +1     
  Misses        314      314           
  Partials      112      112           

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