Release notes 2.6.1

Features

Node availability tracking on UpdateManager: subscribe_to_node_availability / get_node_availability report each node as Available/Unavailable from its status/setup frames. A bare {"sync_status": "lost"} frame arms a uniform 6 s grace timer (configurable via unavailable_delay, matching the vendor app's unreachable window); any ok frame reports Available again.
Write-confirmation tracking: expect_write_confirmation (knob write_confirm_timeout, 6 s) opens a window after each accepted write. If no ok frame arrives, a one-shot discriminating GET decides whether the write was applied (node alive) or silently dropped (node unreachable) — writes to unreachable nodes are ACKed OK with no device ack. Frame taps register lazily and are shared by both features; each enables them independently.

SmartboxValidationError now carries the offending wire payload as .payload, so callers can inspect the actual shape (e.g. distinguish a documented dead-node status frame from other drift).

Both features are backed by live probes from 2026-09-26/30 (api-notes.md, "Node reachability").