Handle Nincada split evolution (Ninjask + Shedinja)

When evolving Nincada, a confirmation prompt now offers to also add
Shedinja as a new encounter on the same route. The Shedinja encounter
uses a "shed_evolution" origin to bypass route-locking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/api/encounters.py

@@ -58,7 +58,7 @@ async def create_encounter(
 
     # Shiny clause: shiny encounters bypass the route-lock check
     shiny_clause_on = run.rules.get("shinyClause", True) if run.rules else True
-    skip_route_lock = data.is_shiny and shiny_clause_on
+    skip_route_lock = (data.is_shiny and shiny_clause_on) or data.origin == "shed_evolution"
 
     # If this route has a parent, check if sibling already has an encounter
     if route.parent_route_id is not None and not skip_route_lock: