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/Cargo.toml

@@ -0,0 +1,32 @@
+[package]
+name = "ssh-handler"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "ssh-handler"
+path = "src/main.rs"
+
+[dependencies]
+windows = { version = "0.58", features = [
+    "Win32_Foundation",
+    "Win32_System_Registry",
+    "Win32_System_LibraryLoader",
+    "Win32_UI_WindowsAndMessaging",
+    "Win32_UI_Shell",
+    "Win32_UI_Controls",
+    "Win32_UI_Input_KeyboardAndMouse",
+    "Win32_System_Threading",
+    "Win32_System_LibraryLoader",
+    "Win32_Security",
+] }
+
+[build-dependencies]
+embed-resource = "2"
+
+[profile.release]
+opt-level = "z"
+lto = true
+codegen-units = 1
+panic = "abort"
+strip = true