fix: replay the builder's held and one-file groups

install.region_drops now skips one-file groups (:file:) and lets no
omitted match displace an installable file, as region.resolve_region_drops
does on the builder side. The property test draws both cases and
compares against the builder.

install.ps1

@@ -12,7 +12,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "376edf794195d74f21e598854b5f3e90fb1955513835264bc9326ad4d2acf9c9"
+    $defaultInstallSha256 = "973d64627af05b239555ab5a83a8902747b7a742bc1176f1b994cf7ac74774f9"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }