[lxsaah]

Description

PR 1 of 3 into feat/352-log-defmt for #352: the groundwork, so that log_*! can forward to defmt in PR 2.

How the scope was found: I temporarily added the defmt branch to the log_*! macros and built every crate that can build with defmt for thumbv7em (core in four feature combinations, MQTT, KNX, serial, TCP). The compiler then listed every argument type without defmt::Format. This PR covers core's share:

Type	Solution
TransportError, CodecError, AuthError, FrameFault	#[cfg_attr(feature = "defmt", derive(defmt::Format))]
DbError	hand-written impl through Display (defmt::Display2Format)
SerializeFailure (pub(crate))	the same, through its existing Display
String, Vec<String>, Arc<str>	core's defmt feature enables defmt/alloc

Why DbError isn't derived: its std-only variants carry io::Error / serde_json::Error, which don't implement Format. The workspace builds docs.rs with all-features = true (Cargo.toml:186), so std and defmt are on together there and a derive would break that build. Display2Format uses the thiserror text, so the defmt line matches the tracing/log line. It isn't interned, which is fine for error paths. This follows the existing hand-written impl for SerializeError (connector.rs).

Guard: a compile-time assertion in lib.rs requires Format for all of these types as soon as defmt (with connector-session) is on, so none can lose it before PR 2 uses them.

No behaviour change: nothing calls these impls until PR 2.

Left for PR 2 (found by the same build): the defmt branch in the macros, the __private re-export, and in aimdb-knx-connector a direct defmt dependency (defmt's macros expand to absolute defmt:: paths) plus defmt/ip_in_core (for core::net::SocketAddr). Also the three std-only crates (zenoh, sync, UDS) need to know the new defmt feature name, because of unexpected_cfgs in #[macro_export] macros.

Verified locally

• All Makefile tests and clippy commands for aimdb-core.
• Every thumbv7em check/clippy command in the Makefile for aimdb-core, aimdb-mqtt-connector, aimdb-knx-connector, aimdb-serial-connector and aimdb-tcp-connector.
• Core with defmt on thumbv7em (alloc,connector-session,remote,defmt, alloc,defmt, alloc,observability,defmt).
• cargo check / cargo doc -D warnings for aimdb-core --all-features (std and defmt together, as on docs.rs).

Related Issue

• Part of [FEAT] Core log macros reach defmt: forward log_*! to defmt and drop the hand-written mirrors #352

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https://claude.ai/code/session_01YPDjQ947yKjLLAq3Xzgu3e