feat: add --events output to the installer

One JSON object per line on stdout (detection, per-file and per-system
check, download progress, summary, exit code) and the usual text on
stderr, so a program driving install.py never parses prose. EVENT_KEYS
is the one schema. The installer page documents it, and a test holds the
two equal. The handheld port reads it.

install.ps1

@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "fe8de4eda0919c268d4d2c0d571c7d0dc1cf675d7604263766f5f99a35aa3724"
+    $defaultInstallSha256 = "3bdaabc9f88e20e09afb99805df5481527611bac796906b98ae6579c3dfcd4a4"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }