Merge main into feat/054-connector-boundary

[lxsaah]

Brings main into the 054 feature branch before stage 9 starts. main adds only #284 (Maximum Packet Size in the embedded CONNECT, with its changelog entries). Please merge with a merge commit, not a squash, so main's commit stays in the feature branch's history.

Commits

1. Merge main into feat/054-connector-boundary. The only conflict was two constants added at the same place in session_loop.rs: PUBACK_ROOM (from feat(mqtt): encode embedded packets into one bbqueue write ring (design 054 §4.7) #283) and MAX_INBOUND_PACKET (from fix(mqtt): advertise Maximum Packet Size in the embedded CONNECT #284). Both are kept. The CONNECT block merged cleanly and now has TopicAliasMaximum(0) and MaximumPacketSize(3328), written through ring.put(&connect, 0).
2. test(mqtt): drop the retained-message proofs now covered by tokio_broker.
   • With fix(mqtt): advertise Maximum Packet Size in the embedded CONNECT #284, the fake broker withholds a push over the client's limit. That makes proof_a_4000_byte_retained_message_reconnects_forever (from feat(mqtt): encode embedded packets into one bbqueue write ring (design 054 §4.7) #283) wrong: it saw 1 connect, not ≥ 3.
   • fix(mqtt): advertise Maximum Packet Size in the embedded CONNECT #284's a_retained_message_over_the_maximum_packet_size_is_withheld covers both the 3,000-byte and the 4,000-byte case, so both retained proofs are removed.
   • proof_an_oversize_publish_is_dropped_silently stays until stage 10.

The test files and changelogs from main are unchanged against main.

Verification

Suite	Passed
embedded-tls lib (includes #284's alignment test and the session-size test with the second CONNECT property)	42
write_ring_proofs	1
session_loop	6
tokio_broker	4
embassy_broker	1
tls_session	3
tls_broker	3
backend_parity	8
--features std	40

• Every aimdb-mqtt-connector clippy leg from the Makefile (host, test targets, thumbv7em) and cargo fmt --all --check: clean.
• embassy-mqtt-connector-demo and weather-station-gamma build for thumbv8m.main-none-eabihf.
• CI does not run on PRs into feat/054-connector-boundary.

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