[lxsaah]

Description

Stage 1 of #391, into the #392 feature branch.

Core

• OutboundLinkBuilder / InboundLinkBuilder gain with_schema(name, version), which reports the record's schema when the link is registered, like RecordRegistrar::schema.
• push_outbound / push_inbound report it before validation, so a rejected link still reports it, as linked_to's schema() call did. A different schema on the same record still conflicts at build().

aimdb-data-contracts

• publish(handle, address) / subscribe(handle, address) return a plain OutboundLink / InboundLink with T's own codec (link_codecs::Default) and T::NAME / T::VERSION as schema.
  • They take a bare address, or a full URL whose scheme matches (#[link_address] keys).
  • Every connector extension (with_qos, with_retain, with_depth, …) applies without new impls, and .with_link_codec(Json) replaces the codec.
  • This uses no wrapper types, which is the choice discussed in [FEAT] publish / subscribe for Linkable contracts, migrate the MQTT demos, document input conventions #391.
• linked_to / linked_from / linked_*_with now register through the same helpers, via reg.output / reg.input. Their behaviour is unchanged, and their existing tests pass untouched.
• Linkable gets #[diagnostic::on_unimplemented]: `X` is not a `Linkable` contract / `X` has no wire format.
• The Linkable and module docs point to publish / subscribe.

Tests

• Core with_schema_reports_the_record_schema:
  • both directions and both forms
  • a rejected link still reports the schema
  • a conflicting schema gives the existing error
• data-contracts publish_and_subscribe_register_like_the_linked_verbs: the same URLs, config and graph schema as linked_to / linked_from, from a bare address and from a full URL.
• MQTT publish_and_subscribe_take_the_knobs: with_qos / with_retain on publish, with_qos on subscribe.
• A compile_fail,E0277 doctest on publish for a record type that isn't Linkable.

Checks

CI doesn't run on PRs into the feature branch, so I ran these locally:

• aimdb-core tests with std,tracing,observability (311) and with alloc
• the thumbv8m build
• aimdb-data-contracts with the full feature set and with --no-default-features --features alloc,linkable
• the MQTT link_ext_tests
• 33 clippy lines from the Makefile for core, data-contracts and MQTT (-D warnings)
• cargo doc with -D warnings for core and data-contracts
• cargo check -p tokio-mqtt-connector-demo

Next

• Stage 2: rewrite mqtt-connector-demo-common and move both demos to connect + publish / subscribe.
• Stage 3: the embassy board module and the B1 PushButton.
• Stage 4: docs.
• Stage 5: changelog.

Related Issue

• Part of [FEAT] publish / subscribe for Linkable contracts, migrate the MQTT demos, document input conventions #391 / [FEAT] reg.input / reg.output with connection handles, so a wrong wiring is a compile error #388 (PR feat: reg.input / reg.output, connector handles and publish / subscribe (#388) #392)

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