Phase 1a: focus spoofing + hook observability

Two problems surfaced in testing: (1) no way to tell whether the injected
hook was actually the input source, and (2) the final design needs the tool
window focused for Steam RPT capture, which would pause/silence games that
react to focus loss. Both are addressed here.

- Focus spoofing (hook/focus_spoof): find the game's main window, subclass it
  to rewrite/swallow WM_ACTIVATE/ACTIVATEAPP/NCACTIVATE/KILLFOCUS, and inline-
  hook GetForegroundWindow/GetActiveWindow/GetFocus to always report the game
  as active. The game keeps running and polling while unfocused.
- Status back-channel (protocol v2): the DLL reports attached/focus-spoof
  flags, game pid/hwnd, a heartbeat, and a cumulative XInputGetState counter.
  The host overlay turns the counter into a live poll rate, so "is the hook
  working" is directly observable.
- Synthetic test-input toggle in the host: forwards a known automated pattern
  (stick circle + periodic A) to prove forwarding independent of the physical
  pad.
- hook_selftest extended to assert the status channel; passes.

Documented the windowed/borderless requirement and the new observable test
flow in the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hook/src/focus_spoof.hpp

@@ -0,0 +1,21 @@
+// Makes the injected game believe it always has foreground focus, so it keeps
+// running and polling input while the tool's window holds the real OS focus
+// (required for Steam RPT to capture the tool). Without this, games that pause
+// or stop polling on focus loss are unusable in the final design.
+#pragma once
+
+#include "ipc_client.hpp"
+
+namespace coop::hook
+{
+
+// Finds the game's main window, subclasses it to suppress deactivation messages,
+// and hooks the focus-query APIs to always report the game as active. Returns
+// true once spoofing is active; safe to retry until the window exists. Reports
+// status through `ipc`.
+bool install_focus_spoof(IpcClient& ipc);
+
+// Restores the original window procedure and removes the focus API hooks.
+void remove_focus_spoof();
+
+} // namespace coop::hook