refactor(routes): move compare domain into routes/compare/ subpackage (#5660)

Slice 2i of the route-domain reorganization (#4082/#4071). Moves
compare_routes.py into routes/compare/, leaving a backward-compat
sys.modules shim at the old path. Pure file reorganization, no behavior
change.

The shim uses sys.modules replacement so the `import ... as cr` +
`monkeypatch.setattr(cr, "SessionLocal", ...)` / `"_owned_endpoint_by_url"`
/ `"_owned_endpoint_by_id"` pattern in test_endpoint_owner_scope_followup.py
reaches the canonical module.

Canonical module imports only from core/, src/, and routes.session_routes
(zero dependency on the legacy shim). One source-introspection test site
repointed: test_endpoint_owner_scope_followup.py (shared with other domains;
only the compare entry repointed here).

Adds tests/test_compare_routes_shim.py to pin the sys.modules shim
contract. Verified: compileall clean; targeted tests pass.

app.py

@@ -787,7 +787,7 @@ from routes.hwfit_routes import setup_hwfit_routes
 app.include_router(setup_hwfit_routes())
 
 # Model A/B Comparison
-from routes.compare_routes import setup_compare_routes
+from routes.compare.compare_routes import setup_compare_routes
 app.include_router(setup_compare_routes(session_manager))
 
 # User Preferences