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/xinput_hook.hpp

@@ -10,7 +10,7 @@ namespace coop::hook
 // Locates the loaded XInput module(s) and hooks the state/capability entry
 // points. `ipc` must outlive the hooks. Returns true if at least one module was
 // hooked. Safe to call repeatedly while waiting for xinput to load.
-bool install_xinput_hooks(const IpcClient& ipc);
+bool install_xinput_hooks(IpcClient& ipc);
 
 // Removes all installed hooks (best effort; used on DLL detach).
 void remove_xinput_hooks();