feat: add component infrastructure

- add focusable runtime state and invalidate entry

- add theme primitives and reusable components

- add component checks for focus and cancellation

test/TinyTUI.ComponentChecks/Program.cs

@@ -0,0 +1,58 @@
+using TinyTUI.Components;
+using TinyTUI.Input;
+using TinyTUI.Rendering;
+using TinyTUI.Text;
+
+var measurer = new TerminalTextMeasurer();
+
+var input = new Input(measurer) { Prompt = "> " };
+AssertFalse(input.Render(20)[0].Contains(CursorMarker.Marker, StringComparison.Ordinal), "unfocused input marker");
+input.Focused = true;
+AssertTrue(input.Render(20)[0].Contains(CursorMarker.Marker, StringComparison.Ordinal), "focused input marker");
+
+var editor = new Editor(measurer) { Placeholder = "type here" };
+AssertFalse(editor.Render(20)[0].Contains(CursorMarker.Marker, StringComparison.Ordinal), "unfocused editor marker");
+editor.Focused = true;
+AssertTrue(editor.Render(20)[0].Contains(CursorMarker.Marker, StringComparison.Ordinal), "focused editor marker");
+
+var list = new SelectList(measurer);
+list.SetItems(["alpha", "beta"]);
+AssertFalse(list.Render(20)[0].Contains(CursorMarker.Marker, StringComparison.Ordinal), "unfocused select marker");
+list.Focused = true;
+AssertTrue(list.Render(20)[0].Contains(CursorMarker.Marker, StringComparison.Ordinal), "focused select marker");
+
+AssertEqual(3, new Spacer(3).Render(10).Count, "spacer line count");
+AssertEqual(0, new Spacer(-1).Render(10).Count, "spacer clamps negative lines");
+
+var truncated = new TruncatedText("abcdef\nignored", measurer)
+{
+    PaddingX = 1,
+    PaddingY = 1,
+};
+var truncatedLines = truncated.Render(6);
+AssertEqual(3, truncatedLines.Count, "truncated text vertical padding");
+AssertEqual(6, measurer.GetWidth(truncatedLines[1]), "truncated text padded width");
+AssertTrue(truncatedLines[1].Contains("abc", StringComparison.Ordinal), "truncated text first line only");
+
+var loader = new CancellableLoader(keybindings: KeybindingRegistry.CreateDefault());
+var canceled = false;
+loader.OnCanceled = () => canceled = true;
+loader.HandleInput(new TuiInputEvent(TuiInputEventKind.Key, KeyNames.Escape));
+AssertTrue(loader.IsCanceled, "cancellable loader token");
+AssertTrue(canceled, "cancellable loader callback");
+
+Console.WriteLine("TinyTUI component checks passed");
+
+static void AssertEqual<T>(T expected, T actual, string name)
+{
+    if (!EqualityComparer<T>.Default.Equals(expected, actual))
+        throw new InvalidOperationException($"{name}: expected '{expected}', actual '{actual}'");
+}
+
+static void AssertTrue(bool condition, string name)
+{
+    if (!condition)
+        throw new InvalidOperationException($"{name}: assertion failed");
+}
+
+static void AssertFalse(bool condition, string name) => AssertTrue(!condition, name);