feat: connectivity redesign phase 2 — instance-aware routing + reference connections by id

apply_routes now iterates over connection instances rather than types:
each instance gets its own fwmark, routing table, interface, and
redirect_port via _routing_connections / _resolve_peer_connection /
_apply_connection_for_src; kill-switch is enforced per iface-instance.
Old per-type MARKS/TABLES constants are kept only as migration scaffolding.

peer_registry: exit_via is now stored as a connection id (or 'default');
_migrate_exit_via_to_connection_id runs on _load_peers to upgrade legacy
type-string values; set_peer_exit_via validates against known connection
ids; VALID_EXIT_VIA removed; config_manager wired in from managers.py.

egress_manager: egress_overrides keyed by service_id → connection_id;
local MARKS/TABLES/EXIT_TYPES/_REDIRECT_PORTS/_add_tor_redirect removed;
(mark, table, redirect_port) resolved at apply-time via
connectivity_manager.get_connection; manifest egress.allowed still
enforced by connection type.

api/app.py + api.js: PUT peer/service exit endpoints accept {connection_id};
back-compat shim resolves a legacy type string to its single active instance.

Tests extended: two same-type instances produce distinct marks/tables/ports;
peer exit_via and egress override id migrations round-trip correctly;
single-instance behaviour is equivalent to the old type-keyed path.

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

tests/test_connectivity_connections.py

@@ -276,7 +276,7 @@ class TestDeleteConnection(_Base):
         res = self.mgr.create_connection('proxy', 'ref', _proxy_cfg())
         cid = res['connection']['id']
         self.peer_registry.list_peers.return_value = [
-            {'peer': 'alice', 'connection_id': cid}]
+            {'peer': 'alice', 'exit_via': cid}]
         out = self.mgr.delete_connection(cid)
         self.assertFalse(out['ok'])
         self.assertIn('in use', out['error'])