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.

tests/test_compare_routes_shim.py

@@ -0,0 +1,25 @@
+"""Regression test for the compare route shim (slice 2i, #4082/#4071).
+
+The backward-compat shim at ``routes/compare_routes.py`` uses ``sys.modules``
+replacement so the legacy import path and the canonical ``routes.compare.*``
+path resolve to the *same* module object. This is required because
+``test_endpoint_owner_scope_followup.py`` uses ``import routes.compare_routes
+as cr`` followed by ``monkeypatch.setattr(cr, "SessionLocal", ...)`` /
+``"_owned_endpoint_by_url"`` / ``"_owned_endpoint_by_id"`` — for those patches
+to take effect at runtime, the legacy module object and the canonical one
+must be identical.
+"""
+
+import importlib
+
+import routes.compare_routes as _shim_compare  # noqa: F401
+
+
+def test_legacy_and_canonical_compare_module_are_same_object():
+    """``import routes.compare_routes`` must alias the canonical module."""
+    legacy = importlib.import_module("routes.compare_routes")
+    canonical = importlib.import_module("routes.compare.compare_routes")
+    assert legacy is canonical, (
+        "routes.compare_routes shim must resolve to the canonical "
+        "routes.compare.compare_routes module object"
+    )

tests/test_endpoint_owner_scope_followup.py

@@ -358,7 +358,7 @@ def test_compare_start_rejects_unowned_endpoint_id(monkeypatch):
 
 
 def test_compare_endpoint_key_lookup_is_owner_scoped():
-    body = Path("routes/compare_routes.py").read_text(encoding="utf-8")
+    body = Path("routes/compare/compare_routes.py").read_text(encoding="utf-8")
     start_body = body.split("def start_comparison", 1)[1].split("# Store comparison record", 1)[0]
     helper_body = body.split("def _owned_endpoint_by_url", 1)[1].split("class RecordVoteRequest", 1)[0]
     id_helper_body = body.split("def _owned_endpoint_by_id", 1)[1].split("class RecordVoteRequest", 1)[0]