fix(relay): preserve sessions through disconnect grace

[wolfy-j]

The last client LEAVE currently sends session shutdown immediately. A short transport interruption can therefore finish the user's sessions before reconnection. This change delays that signal by one second; JOIN cancels the pending signal and resumes the existing plugin. Live updates retain the original disconnect time and reconstruct the remaining grace.

Integrated with #179: reconnect clears both idle retirement and pending disconnect shutdown. The timer stops on reconnect, upgrade and process exit. Client ownership checks and older upgrade snapshots remain supported; exited-child shutdown delivery failures are logged without stopping the hub.

Validation on signed Wippy v0.3.48a:

• Full Relay suite: 47 passing cases, including real live upgrades during pending disconnect with and without reconnect; runtime fixtures require hub and child cleanup.
• Lua lint: 23 entries, clean. Module manifest conventions pass.
• Combined Relay and Session sources: two real Relay -> Session plugin -> Session -> provider -> tool smoke cases store the edit and release hub/session registry names.

Companion: wippyai/session#65 (merged). Deploy with its cooperative turn-drain behavior; #179 handles collection after the hub inactivity timeout. No UI changes. This merge does not release or publish modules.