Refactor timeline to apply SDK diffs directly to NSTableView

[beezly]

Summary

Currently, timeline updates flow through two layers of diffing:

1. LiveTimeline.updateTimeline(diff:) receives granular TimelineDiff operations from the SDK (set, append, insert, remove, etc.) and applies them to the timelineItems array
2. TimelineTableView.updateTimelineItems() receives the full array, compares old vs new IDs, and decides whether to rebuild the NSDiffableDataSourceSnapshot or reload rows in place

This means we discard the SDK diff information and then try to reconstruct it by comparing arrays — re-deriving what we already knew.

Proposed change

Pass the TimelineDiff operations through to TimelineTableView and map them directly to snapshot operations:

SDK Diff	Snapshot Operation
set(index, item)	snapshot.reloadItems([id])
append(values)	Insert at beginning of snapshot (items are reversed)
pushBack(item)	Insert at beginning
pushFront(item)	Append to end
insert(index, item)	snapshot.insertItems with reversed index
remove(index)	snapshot.deleteItems([id])
popFront / popBack	Delete last/first item in snapshot
truncate(length)	Delete items beyond reversed length
reset / clear	Full snapshot rebuild

Benefits

• Correctness: set operations (content-only updates like reactions, read receipts) would reload the exact affected row via snapshot.reloadItems, regardless of visibility — no risk of stale cached views
• Performance: No ID array comparison on every update
• Simplicity: Removes the "are IDs the same?" heuristic

Considerations

• Index reversal: BottomStickyTableView stores items reversed, so SDK indices need transforming (SDK index i in a list of n → table index n - 1 - i)
• Throttling: Diffs are already throttled and batched in listenToTimelineChanges() — batched diffs can be applied sequentially
• Other consumers: timelineItems array on LiveTimeline should remain for other consumers (read receipt marking, reply detail loading, focusEvent)
• The TimelineViewController could subscribe to diffs directly from LiveTimeline (it already holds a reference) rather than going through the SwiftUI representable update cycle

Context

Discussed in #80 — see review comment by @viktorstrate.