BUG : Route accepts `/wiki` but doesnt render the page

[wreckage0907]

A Wiki Space can be created with a route of wiki or wiki/<something>. Such a route can never be served as a public page, so visiting it shows a blank page instead of the wiki content (or a clear error).

/wiki/* is reserved for the editor SPA:

• wiki/hooks.py routes every /wiki/<path> to the SPA (to_route: "wiki").
• The public page renderer explicitly refuses these paths (wiki_document.py: if self.path == "wiki" or self.path.startswith("wiki/"): return False).
• The SPA has no matching route and no catch-all, so it mounts nothing → blank screen.

The conflict is already known internally — wiki/install.py deliberately uses route docs instead of wiki — but nothing stops a user from picking a reserved route.

To Reproduce

1. Create a Wiki Space with route wiki/first-page.
2. Add and publish a page under it.
3. Visit http://<site>/wiki/first-page.

Result: blank page.

Expected behavior

Saving a Wiki Space with route wiki or wiki/<x> should raise a clear validation error instead of silently producing an unreachable page.

Notes

This also affects V2 → V3 migration: the legacy default Space route was wiki, so those spaces hit the same problem.