docs: ADR 0006 — messaging-layer candidate evaluation + backend recommendation

[rxdu]

Completes the research pass requested for ADR 0006 (status stays Proposed).

• Adds a Candidates & evaluation section: seven candidates (iceoryx2, Zenoh, CycloneDDS/FastDDS, eCAL, iceoryx classic, Aeron, dora-rs) scored against the family's criteria — latency, throughput, predictability, lightweight-ness, C++17 ease — with a comparison matrix and per-candidate findings.
• Firms up the backend strategy on the evidence: iceoryx2 intra-host default, Zenoh inter-host, DDS addable behind the seam, ROS 2 remains a bridge.
• Records the Aeron verdict (benchmark reference, not a candidate — ops weight and low-rate tail behavior) and the dora-rs verdict (dataflow framework inverts control per ADR 0005; its data plane validates Zenoh+SHM as libraries).
• Refreshes the open questions (version pinning for pre-1.0 iceoryx2, Zenoh scope, back-pressure surface).