Accept partial ams objects; allow plain-HTTP downloads from a local store on request

A P1P mid-print pushes ams objects with no unit list; every such update was
dropped as "missing field ams", silencing telemetry for the rest of the print
(found on the first real dispatch). The unit list now defaults to empty and a
partial object does not wipe known units.

CONTINUUM_ALLOW_INSECURE_LOCAL_DOWNLOADS=true lets the downloader fetch a
presigned URL over plain HTTP, but only from a loopback or RFC 1918 address.

tests/push_status_parsing.rs

@@ -144,3 +144,13 @@ fn merge_keeps_stage_and_spool_when_a_partial_update_omits_them() {
     assert_eq!(base.stg, Some(vec![54, 14, 13]));
     assert!(base.vt_tray.is_some());
 }
+
+/// A P1P mid-print pushes `ams` objects without the inner unit list. That
+/// used to fail the whole report ("missing field `ams`"), silencing telemetry
+/// for the rest of the print - found on the first real dispatch.
+#[test]
+fn a_partial_ams_object_parses_and_keeps_known_units() {
+    let partial = serde_json::json!({"command":"push_status","sequence_id":"1","msg":1,"gcode_state":"RUNNING","ams":{"tray_now":"2"}});
+    let report: PrintReport = serde_json::from_value(partial).expect("a partial ams object should parse");
+    assert!(report.ams.as_ref().is_some_and(|a| a.ams.is_empty()));
+}