vendor-neutral conformance platform with proc-macro and CLI runner

[fabracht]

Summary

• New mqtt5-conformance-macros proc-macro crate with #[conformance_test] attribute that registers tests into a distributed slice with capability requirements and RFC identifiers
• New mqtt5-conformance-cli binary runner built on libtest_mimic — supports in-process and external SUT testing via --sut descriptor files, with JSON report output
• Migrated all conformance tests from integration test files to the proc-macro system with vendor-neutral SutHandle + TestClient + RawMqttClient abstractions
• Added SUT descriptor profiles for Mosquitto, EMQX, HiveMQ CE, and external mqtt5 broker
• Fixed test isolation issues (unique topic names for shared subscription tests, content-based filtering for $SYS tests)
• Fixed FileBackend::get_session() deadlock caused by Rust 2021 if let temporary lifetime holding read guard across write lock
• Added broker setup guide, conformance profiles, and documentation

Test plan

• All 178 conformance tests pass against in-process broker (0 failures, 3 WebSocket skipped)
• cargo clippy --all-targets --workspace -- -D warnings -W clippy::pedantic passes
• cargo build --workspace succeeds
• Run against external Mosquitto/EMQX to validate SUT descriptor flow