refactor(routes): move history domain into routes/history/ subpackage (#5090)

Slice 2d of the route-domain reorganization (#4082/#4071, per
specs/architecture-runtime-inventory.md §6.3). Moves history_routes.py into
routes/history/, leaving a backward-compat sys.modules shim at the old path.
Pure file reorganization, no behavior change.

The shim uses sys.modules replacement (same pattern as the merged gallery
#4903, research #4975, and memory #5007 slices) so that `import
routes.history_routes`, `from routes.history_routes import X`,
`importlib.import_module(...)`, and the `import ... as history_routes` +
`monkeypatch.setattr(history_routes, ...)` pattern used by
test_history_compact_tool_calls.py / test_fork_session_metadata.py all
operate on the same module object the application uses.

The canonical module does NOT depend on the shim — routes/history/
history_routes.py imports only from core/, src/, and routes.session_routes
(a sibling route module whose old import path stays valid via its own shim
when session is migrated later).

Three source-introspection test sites repointed to the new canonical path:
- test_history_db_fallback_hidden.py
- test_history_order_by_timestamp_regression.py
- test_model_helper_owner_scope.py

Adds tests/test_history_routes_shim.py to pin the sys.modules shim contract
(legacy and canonical paths resolve to the same module object; monkeypatch
via legacy alias reaches the canonical module).

Verified: compileall clean; full suite 4351 passed, 3 skipped.

tests/test_model_helper_owner_scope.py

@@ -34,7 +34,7 @@ def test_task_parse_resolves_with_owner_scope():
 
 
 def test_history_compact_resolves_with_owner_scope():
-    body = _function_source("routes/history_routes.py", "compact_session")
+    body = _function_source("routes/history/history_routes.py", "compact_session")
     assert "owner = effective_user(request)" in body
     assert 'resolve_endpoint("utility", owner=owner or None)' in body