feat(agent): v2 secure-session-core Task 6 - full key fidelity

SPEC-002 Phase 1 Task 6, code-reviewed APPROVED (2 rounds), locally verified
(cargo fmt + clippy -D warnings exit 0 + cargo test --workspace 70 pass + build).

- Viewer WH_KEYBOARD_LL hook diverts system combos (Win/Win+R, Alt+Tab, Alt+Esc,
  Ctrl+Esc) to the remote as a full KeyEvent (vk + scan + is_extended + modifiers)
  and suppresses local handling - GATED on the viewer window having focus AND a
  "send system keys" toggle (default on; Pause/Break host-key), so it never bricks
  the technician's local keyboard when unfocused.
- Agent injection via SendInput KEYEVENTF_SCANCODE + correct KEYEVENTF_EXTENDEDKEY
  (right Ctrl/Alt, arrows, nav, Win, NumLock, numpad Divide) - layout-independent,
  extended-key-correct.
- Ctrl+Alt+Del completes through the SAS helper (SYSTEM SendSAS); installer sets
  the SoftwareSASGeneration policy; 3-tier fail-loud (no false success). SAS named
  pipe DACL tightened from NULL/Everyone to Authenticated Users.
- Modifier hygiene: viewer emits key-ups for held Ctrl/Alt/Shift/Win on focus loss
  / close so modifiers never stick on the remote.
- proto: KeyEvent.is_extended = 7 (additive; older agents derive the flag).

Closes Win+R / Ctrl+C-V / Ctrl+Alt+Del / arrows-vs-numpad fidelity. Live on-device
testing is plan Task 8.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/input/mod.rs

@@ -5,6 +5,7 @@
 mod keyboard;
 mod mouse;
 
+pub use keyboard::vk_is_extended;
 pub use keyboard::KeyboardController;
 pub use mouse::MouseController;
 
@@ -56,7 +57,8 @@ impl InputController {
         self.mouse.scroll(delta_x, delta_y)
     }
 
-    /// Press or release a key
+    /// Press or release a key by virtual-key code only (scan code derived from the VK).
+    #[allow(dead_code)]
     pub fn key_event(&mut self, vk_code: u16, down: bool) -> Result<()> {
         if down {
             self.keyboard.key_down(vk_code)
@@ -65,6 +67,30 @@ impl InputController {
         }
     }
 
+    /// Inject a full-fidelity key event (VK + hardware scan code + extended-key flag).
+    ///
+    /// This is the path used for relayed viewer keystrokes so that scan-code injection
+    /// (layout-independent) and the correct `KEYEVENTF_EXTENDEDKEY` flag are applied.
+    pub fn key_event_full(
+        &mut self,
+        vk_code: u16,
+        scan_code: u16,
+        is_extended: bool,
+        down: bool,
+    ) -> Result<()> {
+        self.keyboard
+            .key_event_full(vk_code, scan_code, is_extended, down)
+    }
+
+    /// Release any modifier keys currently held down on the remote.
+    ///
+    /// Invoked when the viewer loses focus or the session ends so a Ctrl/Alt/Shift/Win
+    /// whose key-up never arrived does not stay latched on the remote desktop.
+    #[allow(dead_code)]
+    pub fn release_all_modifiers(&mut self) -> Result<()> {
+        self.keyboard.release_all_modifiers()
+    }
+
     /// Type a unicode character
     #[allow(dead_code)]
     pub fn type_unicode(&mut self, ch: char) -> Result<()> {