Add msg to v1 push_status, and say why v2 has no vt_tray

Two things the live captures showed that the schemas did not say.

`msg` was missing from the v1 push_status schema, which is the one version
where it is load-bearing. v1 pushes only the fields that changed, except for
a pushall-triggered push which is the whole object, and `msg` is how a v1
message self-describes which it is (0 forced/full, 1 automatic/partial). A
consumer that replaces its cached object with a partial push silently unsets
every field that push did not mention, and there is no other way to tell:
push_status does not echo `sequence_id`, so there is no correlation back to
the pushall that asked for it. continuum-proxy has read this field since the
cache was written; the schema just never declared it.

v2 had it with no description. It is informational there - v2 sends the whole
object every time - so it now points at the v1 note rather than repeating it.

The other half is an absence, which a schema cannot express by staying
silent: v2 has no `vt_tray` because a real v2 printer omits the key entirely
rather than sending an empty object. That is recorded as a comment so nobody
adds it for symmetry with v1, because the distinction is the thing: absence
means "not reported", not "no external spool", and it is exactly why
`EXTERNAL_SPOOLS` in continuum-proxy is a constant instead of being counted
from a report.

schemas/bambulab/v2/report/print/push_status.schema.json

@@ -10,6 +10,8 @@
       "properties": {
         "command": { "const": "push_status" },
 
+        "_comment_no_vt_tray": "There is deliberately no vt_tray here. A real v2 printer omits the key entirely rather than sending an empty object, confirmed live, so absence means 'not reported' and not 'no external spool' - do not add it to make this symmetrical with v1. It is why the external-spool count cannot be derived from a report: counting reported spools would give 0 on exactly the dual-nozzle H2C that has two (see common/ams/ams_mapping.schema.json).",
+
         "2D": { "$ref": "./_2d_report.schema.json" },
         "3D": { "$ref": "./_3d_report.schema.json" },
         "ams": { "$ref": "../../common/ams/ams.schema.json" },
@@ -85,7 +87,10 @@
         "mc_stage": { "type": "integer" },
 
         "model_id": { "type": "string" },
-        "msg": { "type": "integer" },
+        "msg": {
+          "type": "integer",
+          "description": "Full-vs-partial indicator, as in v1 (0 forced/full, 1 automatic/partial) - but informational here, because v2 sends the entire object on every push. The v1 schema carries the full note on why a v1 consumer must read it."
+        },
 
         "percent": { "type": "integer" },
         "plate_cnt": { "type": "integer" },