Steam Input as the primary guest-input path (optional, SDK-gated)

Add SteamInputSource: initializes SteamAPI + Steam Input, loads a bundled action
manifest via SetInputActionManifestFilePath (no partner-backend config needed),
and reads the GameControls action set into CoopPadState -- falling back to XInput
per slot, and to pure XInput if Steam isn't available, so the host always runs.

Enabled automatically when the Steamworks SDK is vendored at
third_party/steamworks_sdk/ (auto-detected by CMake; gitignored and never
committed -- the build is XInput-only without it). Stages steam_api64.dll + the
manifest next to the host and builds coop_steam_input_probe (a console smoke test).

Verified: the probe initializes against the live Steam client and enumerates
controllers; the host degrades gracefully when launched standalone. All 5 tests
pass. Reading actual controller state needs a pad bound through Steam Input for
the running (donor) appid, which XInput otherwise covers.

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

tools/steam_input_probe/main.cpp

@@ -0,0 +1,59 @@
+// coop_steam_input_probe -- console smoke test for SteamInputSource. Brings up
+// Steam Input via the shipping host code (no GUI), prints whether it initialized,
+// how many Steam controllers are connected, and a few polled snapshots, then
+// exits. Needs Steam running and an appid context: either launch it under Steam,
+// or drop a steam_appid.txt (e.g. "480") next to the exe. Built only when the
+// Steamworks SDK is vendored.
+
+#include <cstdio>
+
+#include <windows.h>
+
+#include "input/steam_input_source.hpp"
+
+namespace
+{
+std::string manifest_path()
+{
+	char buffer[MAX_PATH] = {};
+	const DWORD len = GetModuleFileNameA(nullptr, buffer, MAX_PATH);
+	std::string path(buffer, len);
+	const std::size_t slash = path.find_last_of("\\/");
+	if (slash != std::string::npos)
+	{
+		path.resize(slash + 1);
+	}
+	return path + "steam_input_actions.vdf";
+}
+} // namespace
+
+int main()
+{
+	coop::SteamInputSource src;
+	const bool ok = src.init(manifest_path());
+	std::printf("init=%d  steam_active=%d  backend=\"%s\"\n", ok ? 1 : 0, src.steam_active() ? 1 : 0,
+	            src.name());
+
+	for (int frame = 0; frame < 20; ++frame)
+	{
+		src.poll();
+		Sleep(50);
+	}
+
+	std::printf("steam_controllers=%d\n", src.steam_controllers());
+	const auto& pads = src.pads();
+	for (std::uint32_t i = 0; i < coop::kMaxPads; ++i)
+	{
+		const coop::PadInfo& p = pads[i];
+		if (p.connected)
+		{
+			std::printf("  slot %u: source=\"%s\" buttons=0x%04X LX=%d LY=%d LT=%u RT=%u\n", i,
+			            p.source.c_str(), p.state.buttons, p.state.thumb_lx, p.state.thumb_ly,
+			            p.state.left_trigger, p.state.right_trigger);
+		}
+	}
+
+	src.shutdown();
+	std::printf("done.\n");
+	return 0;
+}