A single hung ch.Start() (e.g. Telegram GetMe stalled on DNS/TCP) blocks
InstanceLoader.mu and wedges every subsequent Reload(), leaving new
channels stuck in "checking" forever.
Wrap Start() in a goroutine bounded by reloadStartTimeout (90s) so Reload
can move on to the next instance. On timeout, Stop the channel in a
bounded window, record a typed failure for the UI, and asynchronously
drain the late-returning Start to detect channels that ignore context
cancellation.
Critically, pass the caller's ctx — not a timeout-wrapped ctx — into
ch.Start. Channels routinely derive long-running goroutines from the
ctx they receive (Telegram's pollCtx is WithCancel(ctx)). A timeout-
wrapped ctx would silently kill polling 90s after a successful Reload.
Add regression tests covering both the hang-then-timeout path and the
caller-context preservation invariant.
Closes#914
