Telemetry ruled out the obvious SXSMSI causes (msiserver Manual/normal, no
pending reboot, 37 GB free, no stale InProgress MSI), yet 1603 persists on a
clean machine. Add the last two web-documented script-detectable causes to the
state snapshot: Windows Event Log service status, TrustedInstaller start-type,
and the DisableMSI group policy. Also auto-start EventLog if it's not running
(Office C2R install depends on it). If all clean, the remaining cause is
servicing-stack corruption -> DISM /RestoreHealth + sfc.
dfc83fbb0a