Add Rust prototype of the ssh:// protocol handler

Raw Win32 rewrite (windows-rs + a native dialog resource, no GUI
framework) of SshHandler, aimed at eliminating .NET's ~150MB
self-contained payload and CLR startup cost for what's just a
click-triggered dialog + process launch. Release build cross-compiles
to a ~250KB binary. Same behavior as the WinForms version: registry
setup mode, target parsing/editing dialog, legacy-algorithm option,
launches ssh.exe via ShellExecuteW.

Verified functionally under Wine (registry-setup prompt and the
connect dialog both render and round-trip correctly); real Windows
cold-start numbers still need to be gathered on actual hardware.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GYCT2RpUjFfBfp6CD4rREw

SshHandlerRs/resources/resource.h

@@ -0,0 +1,4 @@
+#define IDI_APPICON       102
+#define IDD_CONNECT       101
+#define IDC_EDIT_TARGET   1001
+#define IDC_CHECK_LEGACY  1002

SshHandlerRs/resources/ssh_handler.rc

@@ -0,0 +1,17 @@
+#include <windows.h>
+#include "resource.h"
+
+IDI_APPICON ICON "ssh_icon.ico"
+
+IDD_CONNECT DIALOGEX 0, 0, 260, 92
+STYLE DS_MODALFRAME | DS_CENTER | WS_POPUP | WS_CAPTION | WS_SYSMENU
+CAPTION "SSH Connect"
+FONT 9, "Segoe UI"
+BEGIN
+    LTEXT           "Target:", -1, 10, 13, 240, 10
+    EDITTEXT        IDC_EDIT_TARGET, 10, 25, 240, 14, ES_AUTOHSCROLL
+    CONTROL         "Legacy mode (older/insecure algorithms)", IDC_CHECK_LEGACY,
+                    "Button", BS_AUTOCHECKBOX | WS_TABSTOP, 10, 47, 240, 12
+    DEFPUSHBUTTON   "Connect", IDOK, 94, 68, 75, 16
+    PUSHBUTTON      "Cancel", IDCANCEL, 174, 68, 75, 16
+END