feat(diag): UI-thread hang watchdog with minidump capture (#120)

The Windows freeze in #120 never reproduces locally and a hung app
prints nothing, so terminal runs and logs came back empty. A watchdog
thread now watches a heartbeat the UI thread bumps on every processed
message (plus a 5 s liveness timer); 25 s of silence writes a minidump
with every thread's stack to %LOCALAPPDATA%\OpenCADStudio plus a note
telling the reporter to attach it to the issue. One dump per run,
armed only after the first beat, suspend-aware, OCS_NO_WATCHDOG=1
disables. Windows-only; windows-sys gains the Debug/Threading binding
features (no new crates).

Co-Authored-By: Claude Fable 5 <[email protected]>

Cargo.toml

@@ -86,7 +86,7 @@ lyon_tessellation = "1.0.20"
 #                       (IApplicationAssociationRegistrationUI dialog).
 # Win32_System_Registry — HKCU registration that lists the app under
 #                       "Open with" for the portable .exe.
-windows-sys = { version = "0.61", features = ["Win32_UI_Shell", "Win32_UI_WindowsAndMessaging", "Win32_System_Com", "Win32_System_Registry", "Win32_Foundation"] }
+windows-sys = { version = "0.61", features = ["Win32_UI_Shell", "Win32_UI_WindowsAndMessaging", "Win32_System_Com", "Win32_System_Registry", "Win32_Foundation", "Win32_System_Diagnostics_Debug", "Win32_System_Threading", "Win32_Storage_FileSystem", "Win32_System_Kernel", "Win32_System_Memory"] }
 
 [patch.crates-io]
 # The `acadrust` crate now tracks the OpenAEC-Foundation/acadifc fork's main