A browser navigation (Sec-Fetch-Mode navigate, or GET/HEAD without it; no
X-Requested-With; Accept naming text/html above every JSON type; not /api,
/_<name> or the admin prefix) that hits a framework-generated 404, 405 or
500 gets a self-contained SummerCMS page: unmatched routes and wrong
methods at the ServeMux, failed Where/WhereIn constraints, overlap-family
misses, the unpublished uploads route and recovered panics. Every other
request keeps the exact stdlib text, wire's opaque JSON 500 or the bare
raw-group 500.
Details (method, path, X-Request-Id) only with app.debug or outside
production. Applications replace the page with a published
surf.ErrorPageRenderer; http.error_pages.enabled false turns it off.
- compile groups routes ServeMux refuses side by side into overlap families
and registers each under one generated method-less pattern
- the family handler tries members in registration order on literals and
Where constraints, sets their path values and runs their own wrapped chain
- no match answers the bare 404; a method mismatch answers ServeMux's 405
and Allow for the same table without the overlap
- unsupported shapes (same shape, {name...}, shadowing route) fail at boot
- README and docs/services/routing.md describe the behaviour
