startRelaunchWaiter returned undefined for four different failures
(staging dir, script copy, PowerShell spawn, missed handshake), and the
detached script runs with stdio ignored. The Store updater then showed
only "Could not register automatic relaunch" and desktop.log had no
reason, so a user report could not be told apart.
Each failure path now writes its cause to desktop.log (error text,
paths, exit code or signal, elapsed ms). The script appends its own
steps and exit reason to %TEMP%\hermes-relaunch-waiter.log, which
survives the stage dir it deletes on exit, and a failed handshake
includes that file's tail in desktop.log.
The two waiter call sites shared the same options block; they now go
through one startUpdateRelaunchWaiter so the log hook cannot be
forgotten at one of them.
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