Add pushall + print_status cache; fix envelope, race, and matching bugs

PrintReport now flattens CommandEnvelope (sequence_id/command) -
push_status.schema.json composes the shared envelope like every other
Bambu message; the struct's old doc comment concluded otherwise from a
capture that turned out to be an incomplete example (also missing dozens
of other real fields), not proof the message lacks them.

Added BambuMqttSession's PrintStatusCache: the in-memory
print.push_status object, replaced wholesale on v2 (always full) or on a
v1 message with msg != 1 (forced/full), merged field-by-field otherwise
(v1's organic partial updates) via PrintReport::merge_from. Tracks
last_sequence_id to warn on a gap in the organic stream. BambuPrinter
gets pushall() (v1-only; pushing.pushall doesn't exist in v2's schema at
all) and print_status().

Three real bugs found and fixed via live testing against an actual P1P,
none of them guessable from the schema alone:
- pushall's reply doesn't echo the sequence_id we send - the printer
  stamps its own free-running counter regardless of what triggered it.
  request()'s ID correlation silently can't work here; first fix attempt
  added request_next_of_category (wait for the next message under a
  category, no ID needed).
- That undershot too: matching by category alone let an unrelated
  "print" message (that category carries many commands - gcode_line,
  pause, push_status, ...) satisfy the wait before the real push_status
  reply arrived, so pushall() returned Ok having consumed the wrong
  message - now keyed by (category, command).
- A real ordering race: handle_incoming_report woke a waiting oneshot
  before updating the print_status cache, so a caller could resume and
  check print_status() before the write landed - pushall() returned Ok
  immediately followed by print_status() still showing None. Fixed by
  applying the cache update before sending on any matched oneshot.

Superseded by real protocol knowledge before any of the above needed to
ship, though: push_status carries its own msg field (1 = automatic/
partial, 0 = forced/full) - the actual, reliable, self-describing signal
for which kind a message is, regardless of timing or which client (if
any) triggered it. PrintStatusCache now reads that directly instead of
inferring it from request correlation, and pushall() is plain fire-and-
forget (publish, no wait) since there's nothing left to correlate for -
confirmed live: msg reads correctly off a real P1P, and the cache
populates/updates correctly with no gap warnings across several organic
pushes. request_next_of_category is kept (a future case may still need
"no correlation ID at all" for real), just no longer used here.

Also: the poll loop now logs when it exits instead of returning silently
- that silence is exactly what made a real dropped MQTT connection to a
live H2C indistinguishable from "nothing happening" during testing.

examples/bambu_commands_demo.rs

@@ -36,8 +36,14 @@ fn main() {
     // --- print report: arrives under "print", NOT "pushing" (pushall
     // triggers this stream, it isn't a direct reply to it) - includes the
     // full ams/xcam example, with one empty tray and one loaded tray, to
-    // exercise the untagged Empty/Loaded split.
+    // exercise the untagged Empty/Loaded split. sequence_id/command added
+    // by hand here (the original real capture this was built from was
+    // itself a truncated example that happened to omit them, along with
+    // dozens of other real fields the schema defines - not proof the real
+    // message lacks them, see this struct's own doc comment).
     let real_print_report = r#"{
+        "sequence_id": "1",
+        "command": "push_status",
         "ams": {"ams": [{"humidity":"4","id":"0","temp":"22.7","tray":[
             {"id":"0"},
             {"bed_temp":"0","bed_temp_type":"0","cols":["000000FF"],"drying_temp":"0","drying_time":"0","id":"1","nozzle_temp_max":"240","nozzle_temp_min":"190","remain":0,"tag_uid":"0000000000000000","tray_color":"000000FF","tray_diameter":"0.00","tray_id_name":"","tray_info_idx":"GFA00","tray_sub_brands":"","tray_type":"PLA","tray_uuid":"00000000000000000000000000000000","tray_weight":"0","xcam_info":"000000000000000000000000"}