Add per-backend input debug visualization + round-trip view

Makes it possible to isolate an input->tool problem from a tool->game one in the
Controllers panel (under Debug details):
- which backend fed each slot is already shown via the per-pad source tag
  ("XInput #0" / "Steam Input") in the Incoming section;
- a new "Round-trip" table shows, per slot, the state we forwarded (the active
  backend's pad) next to what the game actually read back, highlighting matches.

For the round-trip, the XInput hook now echoes the state it returns to the game into
the status (protocol v9->v10: per-slot read_state in HookStatus).

Verified: hook_selftest (x64 + x86) asserts the hook records the game-read state;
full build x64 + x86 clean; ctest x64 9/9, x86 3/3.

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

common/include/coop/protocol.hpp

@@ -12,7 +12,7 @@ namespace coop
 
 // Bump whenever the layout of SharedBlock or CoopPadState changes. The hook
 // refuses to attach to a host with a mismatched version.
-inline constexpr std::uint32_t kProtocolVersion = 9;
+inline constexpr std::uint32_t kProtocolVersion = 10;
 
 // 'COOP' little-endian, used to sanity-check the mapping before trusting it.
 inline constexpr std::uint32_t kProtocolMagic = 0x504F4F43u;
@@ -133,6 +133,10 @@ struct HookStatus
 	// other diagnostics -- benign cross-process races are fine.
 	std::uint16_t rumble_left[kMaxPads];
 	std::uint16_t rumble_right[kMaxPads];
+
+	// The last pad state the hook actually returned to the game per slot, so the host
+	// can show a true input round-trip (forwarded vs what the game read).
+	CoopPadState read_state[kMaxPads];
 };
 
 // Host -> hook control channel. The host requests which hook subsystems should be