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>

host/assets/steam_input_actions.vdf

@@ -0,0 +1,84 @@
+// Steam Input In-Game Actions (IGA) manifest for CoopAllTheThings.
+//
+// SteamInputSource points Steam Input at this file via
+// ISteamInput::SetInputActionManifestFilePath, so the host doesn't depend on a
+// partner-backend-registered config. It declares one action set, "GameControls",
+// with the standard gamepad buttons + two sticks + two triggers. The action names
+// here must match host/src/input/steam_input_source.cpp.
+"In Game Actions"
+{
+	"actions"
+	{
+		"GameControls"
+		{
+			"title"		"#Set_GameControls"
+			"legacy_set"	"1"
+
+			"StickPadGyro"
+			{
+				"LeftStick"
+				{
+					"title"		"#Action_LeftStick"
+					"input_mode"	"joystick_move"
+				}
+				"RightStick"
+				{
+					"title"		"#Action_RightStick"
+					"input_mode"	"joystick_move"
+				}
+			}
+
+			"AnalogTrigger"
+			{
+				"LeftTrigger"	"#Action_LeftTrigger"
+				"RightTrigger"	"#Action_RightTrigger"
+			}
+
+			"Button"
+			{
+				"A"		"#Action_A"
+				"B"		"#Action_B"
+				"X"		"#Action_X"
+				"Y"		"#Action_Y"
+				"LB"		"#Action_LB"
+				"RB"		"#Action_RB"
+				"Back"		"#Action_Back"
+				"Start"		"#Action_Start"
+				"LStickClick"	"#Action_LStickClick"
+				"RStickClick"	"#Action_RStickClick"
+				"DPadUp"	"#Action_DPadUp"
+				"DPadDown"	"#Action_DPadDown"
+				"DPadLeft"	"#Action_DPadLeft"
+				"DPadRight"	"#Action_DPadRight"
+				"Guide"		"#Action_Guide"
+			}
+		}
+	}
+
+	"localization"
+	{
+		"english"
+		{
+			"Set_GameControls"	"Game Controls"
+			"Action_LeftStick"	"Left Stick"
+			"Action_RightStick"	"Right Stick"
+			"Action_LeftTrigger"	"Left Trigger"
+			"Action_RightTrigger"	"Right Trigger"
+			"Action_A"		"A"
+			"Action_B"		"B"
+			"Action_X"		"X"
+			"Action_Y"		"Y"
+			"Action_LB"		"Left Bumper"
+			"Action_RB"		"Right Bumper"
+			"Action_Back"		"Back"
+			"Action_Start"		"Start"
+			"Action_LStickClick"	"Left Stick Click"
+			"Action_RStickClick"	"Right Stick Click"
+			"Action_DPadUp"		"D-Pad Up"
+			"Action_DPadDown"	"D-Pad Down"
+			"Action_DPadLeft"	"D-Pad Left"
+			"Action_DPadRight"	"D-Pad Right"
+			"Action_Guide"		"Guide"
+		}
+	}
+}