fix(router): preserve statusCode of loadable notFound pages

[dangreen]

getViewRefGetter returned the loadable ref loader directly and dropped the extra page module fields, so notFound(loadable(...)) rendered with status 200 instead of 404.

• sync wrapper merges the extra module fields over the loaded ref
• unit test: notFound(loadable(...)) keeps statusCode: 404 before and after the module loads

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.86%. Comparing base (b6ce87b) to head (d04eef5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #166   +/-   ##
=======================================
  Coverage   83.86%   83.86%           
=======================================
  Files         138      138           
  Lines        2968     2969    +1     
  Branches      560      560           
=======================================
+ Hits         2489     2490    +1     
  Misses        340      340           
  Partials      139      139           

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