The macOS launcher fast path gates on hermes_is_installed(), which needs
.hermes-bootstrap-complete next to a built desktop app. Nothing in the Rust
bootstrap pipeline ever wrote that marker -- only install.ps1 did -- so every
reopen of /Applications/Hermes.app re-ran setup instead of launching.
Publish the marker atomically (temp sibling + fsync + rename) because
hermes_is_installed() only checks existence: a torn direct write would arm
the fast path against a half-installed tree. A marker write failure emits
BootstrapEvent::Failed so the installer UI leaves the progress state.
Co-authored-by: giggling-ginger <giggling-ginger@users.noreply.github.com>
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