feat: configurable hotkey binding and ModifierKeyTracker

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Tests/SstCoreTests/HotkeyBindingTests.swift

@@ -0,0 +1,32 @@
+import Testing
+@testable import SstCore
+
+struct HotkeyBindingTests {
+    @Test func allowedBindingsMatchTheSpec() {
+        #expect(HotkeyBinding.allowed.map(\.id) == ["fn", "rightOption", "rightCommand", "rightControl", "leftControl"])
+        #expect(HotkeyBinding.allowed.map(\.keyCode) == [63, 61, 54, 62, 59])
+    }
+
+    @Test func idsAndKeyCodesAreUnique() {
+        #expect(Set(HotkeyBinding.allowed.map(\.id)).count == HotkeyBinding.allowed.count)
+        #expect(Set(HotkeyBinding.allowed.map(\.keyCode)).count == HotkeyBinding.allowed.count)
+    }
+
+    @Test func lookupByKeyCodeAndId() {
+        #expect(HotkeyBinding.binding(keyCode: 61) == .rightOption)
+        #expect(HotkeyBinding.binding(id: "leftControl") == .leftControl)
+    }
+
+    @Test func unsupportedKeysAreRejected() {
+        #expect(HotkeyBinding.binding(keyCode: 56) == nil)   // left shift
+        #expect(HotkeyBinding.binding(keyCode: 57) == nil)   // caps lock
+        #expect(HotkeyBinding.binding(id: "nope") == nil)
+    }
+
+    @Test func everyBindingHasAFlagAndName() {
+        for binding in HotkeyBinding.allowed {
+            #expect(binding.flagRawValue != 0)
+            #expect(!binding.displayName.isEmpty)
+        }
+    }
+}

Tests/SstCoreTests/ModifierKeyTrackerTests.swift

@@ -0,0 +1,18 @@
+import Testing
+@testable import SstCore
+
+struct ModifierKeyTrackerTests {
+    @Test func emitsPressedThenReleasedOnlyOnTransitions() {
+        var tracker = ModifierKeyTracker()
+        #expect(tracker.process(isDown: false) == nil)
+        #expect(tracker.process(isDown: true) == .pressed)
+        #expect(tracker.process(isDown: true) == nil)
+        #expect(tracker.process(isDown: false) == .released)
+        #expect(tracker.process(isDown: false) == nil)
+    }
+
+    @Test func freshTrackerStartsReleased() {
+        var tracker = ModifierKeyTracker()
+        #expect(tracker.process(isDown: true) == .pressed)
+    }
+}