Accept a telemetry stage, and prove the field is really there

`stage` comes from the shared schema, so this is contract tests only.

The test that matters asserts a *wrongly typed* stage is rejected rather than
only that a valid one is accepted. `PrinterTelemetrySchema` permits unknown
properties - verified empirically, `{ totally_bogus: 1 }` validates - so
`{ stage: 54 }` passes whether or not the field is declared, and a test built
only on the positive case would keep passing if someone deleted it.

Worth knowing separately: that open `additionalProperties` means a gateway
sending a misspelled field name is accepted and the value silently dropped,
which is the same drift this suite exists to catch. Not changed here, because
tightening it needs a check against what real gateways send.

tests/edge_protocol.test.ts

@@ -54,6 +54,41 @@ describe("gateway -> backend frames", () => {
     expect(printers.every((p) => typeof p.printer_id === "string" && p.printer_id.length > 0)).toBe(true);
   });
 
+  it("accepts a stage, including stage 0", () => {
+    // `stage` is Bambu's `stg_cur`: what the printer is doing when it is not
+    // laying down plastic. 0 means printing, and a schema or serialiser that
+    // treats 0 as absent loses the distinction that makes the field worth
+    // having - a printer at 14% and layer 0 is mid-bed-levelling at stage 54
+    // and genuinely stuck at stage 0.
+    for (const stage of [0, 13, 54]) {
+      check(EdgeClientMessageSchema, {
+        type: "telemetry",
+        printers: [{ printer_id: "p1p", name: "P1P", percent: 14, layer: 0, stage }],
+      });
+    }
+
+    // The assertion that makes the loop above mean anything. This schema
+    // permits unknown properties, so `{ stage: 54 }` validates whether or
+    // not `stage` is declared - verified, it does. Rejecting a *wrongly
+    // typed* stage is what proves the field is really in the schema, so this
+    // is what fails if someone removes it.
+    expect(
+      Value.Check(EdgeClientMessageSchema, {
+        type: "telemetry",
+        printers: [{ printer_id: "p1p", name: "P1P", stage: "54" }],
+      }),
+    ).toBe(false);
+  });
+
+  it("still accepts telemetry with no stage at all", () => {
+    // The field is additive: the committed live capture predates it, and a
+    // gateway that has not been updated yet simply omits it.
+    check(EdgeClientMessageSchema, {
+      type: "telemetry",
+      printers: [{ printer_id: "p1p", name: "P1P", percent: 0 }],
+    });
+  });
+
   it("rejects a telemetry batch whose printer has no id", () => {
     // printer_id and name are the only required fields; everything else is
     // genuinely unknowable for a printer that has only just connected.