feat(simnet): add SimNet protocol end-to-end support

- model: SimNet protocol fields + NormalizeSimnet; type+port protocol uniqueness
- server config: OmnXT runtime config delivery via compatible() simnet case
- credentials: derive per-user psk/key_id from subscription (pkg/simnet), no new table
- subscription: adapter buildOmnxtSimnetConfigs + base64 buildOmnxtProtocolLinks + OmnXT SimNet application (migration 02161)
- UA gating: hide experimental protocols from non first-party clients (download + JSON node-list)
- admin: normalize simnet on create/update and on GET responses
- tests: 21 simnet unit tests; full suite green

internal/logic/server/getServerUserListLogic_test.go

@@ -38,6 +38,7 @@ func TestNormalizeServerUserListProtocol(t *testing.T) {
 		{"tuic unchanged", "tuic", "tuic"},
 		{"shadowsocks unchanged", "shadowsocks", "shadowsocks"},
 		{"anytls unchanged", "anytls", "anytls"},
+		{"simnet unchanged", "simnet", "simnet"},
 		{"empty unchanged", "", ""},
 	}
 	for _, c := range cases {