Apply pushed comment updates without refreshing the whole thread

Problem/Motivation

The alpha push client still refreshes through HTTP. Large audiences can therefore generate a request burst after each notification.

Proposed resolution

Deliver complete authorized comment updates that clients can apply in the correct position. Preserve progressive loading, drafts and selection, with HTTP reconciliation when an update cannot be safely applied.

Remaining tasks

Implement payload preparation and client application. Test inserts, edits, visibility changes, moves and partially loaded branches.