feat: pos system event-trigger — state-based threshold monitors (eventer)

Each line of event.env is an independent rule: ["msg" if ] <check> <op> <thr>.
Check runs on every pass; first numeric output compared float-safe; op is the
rightmost 'op threshold' pair so checks with their own >/< parse fine. Alerts
once on false->true + one recovery message on true->false (no repeats while
the condition holds); per-rule state keyed by rule-line hash in
~/.local/share/linux_post_install/eventer/state/.

Subcommands: run (timer entrypoint), config (interactive add/remove/edit with
check-validation), list (rules + live values), enable [interval] (systemd
user timer pos-event-trigger.timer; 5m..weekly or OnCalendar; graceful without
a user manager, loginctl enable-linger attempt), disable, status. --dry-run
honors the DEV.md convention. Alerts via lib/notify.sh (Telegram default).

New: bin/pos-system-event-trigger, lib/eventer-lib.sh, config/event.env
template (no-clobber via postinstall), install.sh lib install, INTERACTIVE_CMDS
entry. Docs: POS.md system row, HOWTO.md index, howto/event-trigger.md.
make gen && make check green; functional tests cover trigger/recovery/no-repeat,
float+unit parsing, editor add/remove/edit + validation + dry-run, timer
enable/disable/status, dispatcher routing.

bin/pos

@@ -173,6 +173,9 @@ EXAMPLES
   pos entertainment enable weather 5m     Auto-send weather to Telegram every 5 min
   pos entertainment status                Show enabled plugins + next run
 
+  pos system event-trigger config         Edit threshold rules (event.env)
+  pos system event-trigger enable 5m      Monitor rules on a 5-minute timer
+
   pos ai gemini ask "Explain DNS in one line"
                                           Ask Google Gemini a one-shot question
   pos ai gemini chat                      Interactive multi-turn chat
@@ -241,7 +244,7 @@ MAIN_LOG="$LOG_DIR/pos.log"
 log_cmd() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $* → exit $2" >> "$MAIN_LOG"; }
 
 # Commands that read from stdin interactively — only log invocation
-INTERACTIVE_CMDS="system-firewall media-mp4 system-backup usb-server communication-telegram-listener ai-gemini config"
+INTERACTIVE_CMDS="system-firewall media-mp4 system-backup usb-server communication-telegram-listener ai-gemini system-event-trigger config"
 
 for ((i=n-1; i>=0; i--)); do
     cmd="pos"