Invalidate cached terminal surfaces after daemon reconnection

[richardcase]

Problem

SurfaceHost caches surfaces by pane ID indefinitely. A daemon restart respawns agents with the same IDs, but the cached surface still owns the old, exited clowder attach process. Reconnection therefore leaves the revived agent visually dead.

Evidence: https://github.com/richardcase/clowder/blob/b5492d5/macos/Sources/ClowderApp/SurfaceHost.swift#L28-L50
Reconnect path: https://github.com/richardcase/clowder/blob/b5492d5/macos/Sources/ClowderCore/AppModel.swift#L191-L210

Acceptance criteria

• Associate surfaces with a connection generation.
• Evict/recreate all affected surfaces after same-backend reconnect.
• Refresh selected split trees and add a reconnect test with unchanged pane IDs.