overlay: toast notifications (#704)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds toast notifications to the overlay.

Toast notifications are transient windows (shown for about 4 seconds).
These are drawn on top of the game even when the overlay is inactive
(i.e., when no window is open), and does not cause any input sink
behavior.

Add toasts for some common user-driven actions that could use a
notification, such as:

* screenshot
* card insert
* PIN macro
* API client connect/disconnect
* virtual printer
* screen resize toggle / scene switch

Add an option to change where the toasts are displayed, or to turn it
off entirely.

Add SDK function for it, and add it to the samples.

## Testing
Tested DDR (shown at bottom right, as is for most games) and RB (shown
top right by default).

src/spice2x/overlay/notifications.h

@@ -0,0 +1,55 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+
+namespace overlay::notifications {
+
+    // master switch for the notification system; when false, add() is a no-op.
+    // controlled by selecting "none" for the -notifypos launcher option.
+    extern bool ENABLED;
+
+    enum class Severity {
+        Info,
+        Success,
+        Warning,
+        Error,
+    };
+
+    // screen anchor for the toast stack. toasts stack away from the anchored edge.
+    enum class Position {
+        BottomRight,
+        BottomLeft,
+        TopRight,
+        TopLeft,
+    };
+
+    // current toast anchor. defaults to BottomRight; may be reassigned by
+    // apply_game_default_position() or by the user via -notifypos.
+    extern Position POSITION;
+
+    // apply the default toast position appropriate for a game (by display name,
+    // as returned by eamuse_get_game()). called once after game autodetect, before
+    // any user -notifypos override is applied.
+    void apply_game_default_position(const std::string &game_name);
+
+    // add a notification (thread-safe). returns the assigned id, or 0 if the
+    // notification was dropped (overlay disabled or notifications disabled).
+    uint64_t add(Severity severity, std::string text);
+
+    // rate-limited variant of add(). suppresses the toast if another call with
+    // the same `key` succeeded within the last `cooldown_seconds`. useful for
+    // events that can fire every frame (e.g. a button held down). returns the
+    // assigned id, or 0 if the toast was suppressed or dropped. thread-safe.
+    uint64_t add_throttled(Severity severity, const std::string &key,
+                           double cooldown_seconds, std::string text);
+
+    // true if there is at least one notification that still needs to be drawn.
+    // safe to call from the render thread without locking the underlying store.
+    bool has_pending();
+
+    // draw all active notifications and prune expired ones.
+    // must be called from the ImGui render thread inside a NewFrame/EndFrame pair.
+    void draw();
+}