Carry mDNS interface scope_id for link-local IPv6 sends#13
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Matter devices commonly advertise only a link-local IPv6 operational address (fe80::/10). Sending UDP to such an address requires the interface zone (scope_id); without it sendto fails with EINVAL (IPv4 socket) or No route to host (IPv6 socket), so commissioning/operation never completes. Capture the interface index on which each device's mDNS reply was received (ll_scope map in MdnsService), expose it as MatterDeviceInfo::scope_id, encode it into the address as [fe80::..%idx]:port (discover::addr_string), and apply it as the scope_id when sending (transport split_scope + Connection::scope_id). Incoming datagrams are matched zone-insensitively (scopeless_key).
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Problem
Devices that advertise only a link-local IPv6 operational address (fe80::…) can't be reached: every UDP send fails with EINVAL (IPv4 socket) or No route to host (IPv6 socket), so CASE/commissioning never completes.
Cause
A link-local address needs the interface zone (scope_id) to be sendable — fe80::/64 exists on every interface. The address was taken from mDNS without a zone, so scope_id was 0.
Fix
Carry the scope from where it's known to where it's needed:
mdns2 records the interface each link-local AAAA reply arrived on (MdnsService::scope_for).
MatterDeviceInfo::scope_id is set from it; discover::addr_string encodes [fe80::…%idx]:port.
transport parses the zone (split_scope), stores it on Connection, and applies it as scope_id on send; inbound datagrams are matched zone-insensitively.
The scope comes from the actual receive interface, so it's correct on multi-interface hosts (no guessing). Non-link-local and IPv4 paths are unchanged.
Test
Verified end-to-end against a real accessory (Shelly Presence, Wi-Fi) on macOS: BLE commissioning → CASE → CommissioningComplete, and operational attribute reads