fix: handle monitor hotplug to survive suspend/resume (v0.6.9)

moonlock crashed with segfault in libgtk-4.so after suspend/resume when
HDMI monitors disconnected and reconnected, invalidating GDK monitor
objects that statically created windows still referenced.

Replace manual monitor iteration with connect_monitor signal (v1_2) that
fires both at lock time and on hotplug. Windows are now created on demand
per monitor event and auto-unmap when their monitor disappears.

DECISIONS.md

@@ -2,6 +2,13 @@
 
 Architectural and design decisions for Moonlock, in reverse chronological order.
 
+## 2026-04-09 – Monitor hotplug via connect_monitor signal
+
+- **Who**: Nyx, Dom
+- **Why**: moonlock crashed with segfault in libgtk-4.so after suspend/resume — HDMI monitor disconnect/reconnect invalidated GDK monitor objects, and the statically created windows referenced destroyed surfaces. Crash at consistent GTK4 offset (0x278 NULL dereference), 3x reproduced.
+- **Tradeoffs**: Wallpaper texture now loaded before `lock()` instead of after (connect_monitor fires during lock() and needs the texture). Local JPEG loading is fast enough that the delay is negligible. Shared state moved to Rc's for the signal closure — slightly more indirection but necessary for dynamic window creation.
+- **How**: (1) Bump gtk4-session-lock feature from `v1_1` to `v1_2` to enable `Instance::connect_monitor`. (2) Replace manual monitor iteration with `lock.connect_monitor()` signal handler that creates windows on demand. (3) Signal fires once per existing monitor at `lock()` and again on hotplug. (4) Windows auto-unmap when their monitor disappears (ext-session-lock-v1 guarantee). (5) Fingerprint listener published to shared Rc so hotplugged monitors get FP labels.
+
 ## 2026-03-31 – Fourth audit: peek icon, blur limit, GResource compression, sync markers
 
 - **Who**: Ragnar, Dom