feat: add Nex and Gopher listeners

Reuses md2txt's existing nex and text renderers (both already ship
unmodified) -- rendered unwrapped gemtext still works for Gemini/Spartan
clients that wrap themselves, but Nex and Gopher clients don't, so those
two get the classic 80-column pre-wrap instead.

Neither protocol has a redirect status of its own, so Site.resolve_flat()
follows the /foo.md -> /foo canonical bounce and the WML-card-only ->
parent redirect server-side and hands back real content directly, rather
than leaving the two new listeners to invent a redirect convention that
doesn't exist on the wire.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

devbox.json

@@ -10,7 +10,7 @@
     "scripts": {
       "sync": ["uv sync"],
       "test": ["uv run pytest"],
-      "run": ["uv run smolweb --root ./content --host localhost --spartan :3000"]
+      "run": ["uv run smolweb --root ./content --host localhost --spartan :3000 --gopher :7070"]
     }
   }
 }