Fix NV modem discovery on shared MAVLink links

[Rouniy]

Summary

• register the private SkyComm MAVLink dialect before any shared connection begins reading
• discover NV5 from its private status/configuration messages and NV4 from UAVCAN_NODE_INFO names NV_TX/NV_RX
• remove system/component ID range assumptions and probe every endpoint already observed by each shared link
• replay cached discovery packets when the setup page opens after a modem was already seen
• clarify that NV Modem uses Mission Planner's existing connection and never opens a second port

Verification

• 1132/1132 Release tests passed
• focused NV modem tests: 21/21 passed
• Debug app build: 0 warnings, 0 errors
• changed-file dotnet format verification passed