Add passive mDNS discovery as a second device-discovery source

[chrisdp]

Adds passive mDNS / DNS-SD discovery as a second source alongside the existing SSDP discovery, wired into RokuFinder so it needs no DeviceManager changes.

What it does

• New MdnsListener joins the mDNS multicast group and listens for device announcements without sending any queries. This adds no traffic of its own and sidesteps the responder rate-limiting that makes repeated active scanning unreliable.
• Identifies Rokus by the integrator=Roku TXT marker (or the _display service) and reads the serial from the device's .local hostname, which matches the SSDP USN serial one-to-one.
• RokuFinder translates sightings into its existing found / lost events, deduped by serial. mDNS sightings deliberately bypass the ssdp:alive heartbeat-suppression path, which is tuned to Roku's ~20-minute SSDP cadence and does not apply to mDNS. mDNS sightings also reset the active-scan settle timer so they participate in scans.
• Zero new dependencies: the DNS wire parsing is hand-rolled.

Notes

• mDNS is opportunistic and best-effort: it reliably catches boot/join/change/goodbye and any responses other hosts' queries elicit, but does not guarantee finding a long-idle device. SSDP remains the primary, dependable source; mDNS supplements it and provides richer TXT metadata (model, firmware, friendly name).
• Also adds a project mdns-scan skill (.claude/skills/mdns-scan/): a standalone scanner used during development to inspect what devices advertise over mDNS.

Draft: opening for early review of the approach.