display: coalesce movement updates to prevent notify storms; add head…

[aviralgarg05]

Title: Fix display layout re-entrancy and add headless layout tests #430

Summary

• Problem: On complex multi-monitor arrangements (3+), dragging and auto-alignment could cause redundant GObject notify storms leading to layout resets or, in rare cases, crashes (stack traces implicated move_y/move_x).
• Fix: Batch x/y movement updates and guard geometry changes to minimize re-entrant property notifications.
• Validation: Added a headless test suite for layout logic (overlap resolution, edge adjacency, origin normalization) that passes in container environments without a compositor.

Changes

1. Widgets/DisplayWidget.vala

   • Add move_by(dx, dy): freezes/thaws notify around both deltas; queues one resize.
   • move_x/move_y: early return on zero delta to avoid no-op notifies.
   • set_virtual_monitor_geometry: update only when values actually change; queue resize once.

2. Widgets/DisplaysOverlay.vala

   • Use move_by in drag updates and in alignment/overlap resolution paths to coalesce updates.

3. Tests

   • tests/test_layout.vala: GLib-only tests that simulate multi-monitor layout behavior:
     • resolve overlaps among 3+ displays
     • normalize origin to (0,0)
     • ensure displays remain connected by edges (no isolated enabled displays)
   • Added to Meson and runs with meson test -C build

Risk / Compatibility

• No public API changes; behavior change is internal (coalesced movements and guarded geometry updates).
• Build still emits the same GTK deprecation warnings as before; no new warnings/errors introduced.

Issue coverage

• Addresses the reported instability by preventing re-entrant notify storms during drag/auto-align cycles, which previously could cascade through connected signals.

[aviralgarg05]

please review