[lxsaah]

Description

Stage 3 of 3 for #389 (design 059), into the #392 feature branch.

• MqttLinkExt, MqttOutboundLinkExt (aimdb-mqtt-connector), Ros2LinkExt (aimdb-zenoh-connector) and LinkCodecBuilderExt (aimdb-data-contracts) are implemented for OutboundLinkBuilder<T, R> / InboundLinkBuilder<T, R> rather than the attached aliases. As a result, with_qos, with_retain, with_depth, with_reliability and with_link_codec also work on OutboundLink / InboundLink. Only the impl headers changed; the bodies are the same.
• The MQTT link_ext module docs name the generic builders and add a compiling example of the detached form, using reg.input(..).output(..).
• docs/aimdb-usage-guide.md gets a short "Links as values" section.

Tests

• MQTT detached_links_take_the_same_knobs: detached links with QoS, retain and the Postcard codec record the same config as link_to / link_from, and both take the bounded payload path.
• Zenoh tests/link_ext.rs: depth and reliability are recorded the same way on both forms.
• data-contracts codec_verb_applies_to_detached_links: the codec and its recorded wire format on .output() / .input() links.

Checked locally:

• the full aimdb-data-contracts test command from the design doc
• the MQTT link_ext_tests and doctests
• the new zenoh test

clippy and the demo check are left to CI.

The changelog entries for all three stages follow in one commit on the feature branch before it merges to main.

Related Issue

• Part of [FEAT] Detachable link builders and Input / Output traits in core #389 / [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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