mesh: LightOverrideMsg — command a single light across devices

[bennotk]

Part of LightWitch M1 — see the rework plan. Builds on the local override mechanics from #457.

Proposed change

• New additive MsgType::LightOverride with LightOverrideMsg:
  • targetMac[6] + lightIndex (addressing mirrors SetGroupMsg)
  • full LightConfig snapshot
  • durationMs (0 = permanent until displaced)
  • clear flag (drop the target's override without supplying a snapshot — so any device or, later, a graph node can clear)
• Receiver applies it through the config,web: per-light override with last-command-wins arbitration #457 arbitration (receive order decides between concurrent senders — last wins).
• No changes to any existing message; stays inside the same-firmware wire policy.

Acceptance criteria

• A broadcast with targetMac of device B overrides exactly the addressed light on B; other devices update nothing.
• clear returns the light to its group from any sender.

Checklist obligations

• Add the message to the inventory table in docs/mesh-compatibility.md with its framing rule (fixed-size, len >= sizeof)
• Mock parity where the UI is involved (dashboard reflects an override arriving via mesh)