Step 519: add modifier-edge shortcut notation model

editor/src/ModifierEdgeNotation.h

@@ -0,0 +1,76 @@
+#pragma once
+// Step 519: Modifier-Edge Shortcut Notation
+
+#include <string>
+#include <vector>
+#include <algorithm>
+
+struct ModifierEdges {
+    bool left = false;   // Ctrl
+    bool right = false;  // Shift
+    bool top = false;    // Super/Win
+    bool bottom = false; // Alt
+};
+
+struct ShortcutGlyph {
+    char key = '?';
+    ModifierEdges edges;
+    std::string fallbackText;
+};
+
+class ModifierEdgeNotation {
+public:
+    static ModifierEdges fromModifiers(bool ctrl, bool shift,
+                                       bool superKey, bool alt) {
+        return {ctrl, shift, superKey, alt};
+    }
+
+    static ShortcutGlyph buildGlyph(char key,
+                                    bool ctrl,
+                                    bool shift,
+                                    bool superKey,
+                                    bool alt,
+                                    bool glyphEnabled) {
+        ShortcutGlyph g;
+        g.key = normalizeKey(key);
+        g.edges = fromModifiers(ctrl, shift, superKey, alt);
+        g.fallbackText = fallbackLabel(g);
+        if (!glyphEnabled) {
+            g.edges = {};
+        }
+        return g;
+    }
+
+    static int activeEdgeCount(const ModifierEdges& e) {
+        return (e.left ? 1 : 0) + (e.right ? 1 : 0) +
+               (e.top ? 1 : 0) + (e.bottom ? 1 : 0);
+    }
+
+    static bool compactEnough(const ShortcutGlyph& g) {
+        // Key plus edge markers should fit compact control regions.
+        return g.key != '?' && g.fallbackText.size() <= 20;
+    }
+
+    static std::string fallbackLabel(const ShortcutGlyph& g) {
+        std::vector<std::string> parts;
+        if (g.edges.left) parts.push_back("Ctrl");
+        if (g.edges.right) parts.push_back("Shift");
+        if (g.edges.top) parts.push_back("Super");
+        if (g.edges.bottom) parts.push_back("Alt");
+        std::string out;
+        for (size_t i = 0; i < parts.size(); ++i) {
+            if (i > 0) out += "+";
+            out += parts[i];
+        }
+        if (!out.empty()) out += "+";
+        out += std::string(1, g.key);
+        return out;
+    }
+
+private:
+    static char normalizeKey(char key) {
+        if (key >= 'a' && key <= 'z') return static_cast<char>(key - 'a' + 'A');
+        if ((key >= 'A' && key <= 'Z') || (key >= '0' && key <= '9')) return key;
+        return '?';
+    }
+};