feat: explain check refusals, install offline

--check names why each flagged file is refused and where each core
opens it, and names a pack part missing from the folder from the
markers split_pack writes. --from-pack installs from a downloaded
pack or its parts with no network request, --manifest reads a saved
file list, both checking every hash. The end of a run names the
system or core that pulls most of the download when narrowing saves
over half.

install.ps1

@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "ad3b338a8f709be45352a12bd9c6a6250819a8fb9afd19498e24f41e3f062d20"
+    $defaultInstallSha256 = "996cf4a8e21b8f05a524778046d0d32c807fbe3564f6edefdf5bee1add9c76cd"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }