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.sh

@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="fe8de4eda0919c268d4d2c0d571c7d0dc1cf675d7604263766f5f99a35aa3724"
+DEFAULT_INSTALL_SHA256="3bdaabc9f88e20e09afb99805df5481527611bac796906b98ae6579c3dfcd4a4"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {