fix(native): eliminate explorer hang deadlock and improve taskbar crash recovery

[fix2it]

Background & Context

While using the app on Windows 11, I ran into the issue discussed in #94 during an explorer.exe UI hang (Event ID 1002, Application Hang). The app vanished from the taskbar and froze completely. After digging into the root cause, I found that while the recent context-menu fixes in v2.10.24/25 addressed menu deadlocks, a couple of underlying Win32 LPC and watchdog checks were still causing the monitor thread to hang indefinitely or fail to re-dock when Explorer hung or restarted. Here is the detailed breakdown and the fix.

Problem

When explorer.exe experiences a UI hang or freeze (Event ID 1002, Application Hang), Claude-Code-Usage-Monitor freezes indefinitely and vanishes from the Windows 11 taskbar without recovering:

1. Deadlock on Explorer hang: SHAppBarMessage(ABM_GETTASKBARPOS, ...) in src/native_interop.rs relies on synchronous LPC with Explorer's UI thread. When Explorer is unresponsive, this call blocks indefinitely, freezing the monitor thread.
2. Watchdog failure on taskbar crash: When Explorer crashes or restarts, Windows does not automatically destroy cross-process child windows. IsWindow(child_hwnd) continues to return true, causing spawn_taskbar_watchdog to believe the host taskbar is still alive and leave the widget orphaned.
3. Redocking latency: Redocking was delayed until the periodic watchdog timer ticked instead of immediately responding to shell rebirth.

Solution

• Non-blocking taskbar metrics: Replaced SHAppBarMessage with get_window_rect_safe(taskbar_hwnd). Non-blocking GetWindowRect takes ~0.003 ms and never deadlocks during Explorer hangs (verified empirically via NtSuspendProcess tests where SHAppBarMessage hung for >1500 ms).
• Robust watchdog parent verification: In spawn_taskbar_watchdog, we now check GetParent(hwnd). If a surface has a parent assigned and !IsWindow(parent), the watchdog detects the dead host and triggers redocking. Unparented/floating surfaces safely evaluate to false to avoid false-positive restart loops. Uses a zero-heap lazy iterator (once(...).chain(...).any(...)).
• Instant recovery via TaskbarCreated: Explicitly triggers render_layered() when the TaskbarCreated broadcast message is received.

Testing

• Explorer Freeze: Simulated via NtSuspendProcess on explorer.exe: monitor remained 100% responsive (Responding = True, ~28 MB RAM) and resumed immediately.
• Graceful Shell Restart: Tested via WM_CLOSE to Shell_TrayWnd and restarting explorer.exe: widget cleanly and automatically redocked to the new taskbar (Visible = True).
• Abrupt Shell Crash: Tested via taskkill /f /im explorer.exe: watchdog cleanly detected dead parent and recovered with single-instance mutex handoff.
• Single-Instance Guard: Verified that launching a second instance exits with code 0 without disturbing the running monitor.
• Unit Tests & CI: All 200+ unit tests pass cleanly (test result: ok).

---

Note: This fix was researched, profiled, and refined in collaboration with an AI coding agent (Google Antigravity), and rigorously validated across live Windows 11 shell hang & restart lifecycles.