- cmd_gui with DESKTOP_STARTUP_ID: the entry is installed only after the fake Electron
writes the reveal byte to HERMES_DESKTOP_READY_FD (spawn precedes install).
- cmd_gui without DESKTOP_STARTUP_ID (control): install still precedes the spawn.
- DeferredDesktopEntryInstall.finish(): an app that exits without revealing heals
exactly once, after the exit.
- linux-launcher-ready: one byte, fd closed, variable consumed; garbage/absent = no-op.
Docs: user-guide/desktop.md explains when the entry write happens for grid launches.
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