Fix crash on invalid geometry from NVIDIA multi-monitor setups

[appositeit]

Problem

NVIDIA multi-monitor setups can report invalid intermediate geometries (e.g. 8x8 pixels) during mode switches and DPMS wake. This causes:

1. Screensaver crash when the stage tries to render with invalid monitor geometry, leading to Gtk widget allocation failures
2. Orphaned PAM helpers — when the screensaver crashes, cinnamon-screensaver-pam-helper processes (~12MB each) are reparented to PID 1 and accumulate over time

Solution

Commit 1: Validate geometry during monitor change events

• C library (cs-screen-x11.c): Add has_valid_monitor_dimensions() check with rollback to previous valid state and a 500ms deferred re-check when invalid geometry is detected
• Python stage (stage.py): Add MIN_MONITOR_DIMENSION (64px) checks before refreshing monitor views, with deferred re-check on invalid geometry
• Both on_monitors_changed and on_screen_size_changed handlers are guarded in both C and Python layers

Commit 2: Add exception handler and PAM helper cleanup on exit

• sys.excepthook for proper traceback logging on unhandled exceptions
• atexit handler to kill child PAM helper processes
• SIGTERM handler for graceful cleanup

Testing

Tested on a single machine:

• Ubuntu 25.10 (Cinnamon 6.4, cinnamon-screensaver 6.4.0)
• NVIDIA driver 570.133.07, proprietary
• Dual monitor: 3440x1440 (DP) + 2560x1440 (HDMI)
• DPMS sleep/wake cycling over multiple days with no screensaver crashes and zero orphaned PAM helpers

Previously this configuration would crash the screensaver during DPMS wake events. With these guards, invalid geometry is detected and deferred until the driver settles.

This has only been tested on one machine and has not been widely tested.

Disclosure

These patches were developed with AI assistance (Claude Code). The changes have been manually reviewed.

Related issues

• Removing or Adding a Screen During Standby crashes Cinnamon-Screensaver on Resume #298 — Removing or adding a screen during standby crashes screensaver on resume
• Crashes after suspend resume with changed monitor configs #414 — Crashes after suspend/resume with changed monitor configs
• Hard crash when closing laptop lid while external monitors are attached via USB-C #468 — Hard crash when closing laptop lid while external monitors are attached via USB-C
• cinnamon-screensaver memory usage increasing significantly over time #72 — Memory usage increasing significantly over time (PAM helper cleanup partially addresses this)