[minor] Add CropOverlay, a crop rectangle drawn over an ImageCanvas

CLAUDE.md

@@ -28,7 +28,7 @@ This is the **ktsu ImGui Suite**, a collection of .NET libraries for building De
 ### Libraries
 
 - **ImGui.App** (`ktsu.ImGui.App`) - Application foundation with windowing, rendering, font/texture management, PID frame limiting, DPI awareness. Image decoding is self-contained (`ImGui.App/Images/`), so the package carries no imaging dependency; see [Image decoding](#image-decoding) below.
-- **ImGui.Widgets** (`ktsu.ImGui.Widgets`) - Custom UI components, grouped as the README's feature list groups them: input and controls (`Switch`, `SegmentedControl`, `Stepper`, `TimecodeField` over the ImGui-free `Timecode`/`TimecodeRate`, `RangeSlider`, `ChannelFader`/`FaderTaper`, `XYPad`, `ColorWheel`/`LiftGammaGain`, `Knob`/`KnobWithDrag`, `Rating`, `Chip`/`ChipGroup`, `PinInput`, `SearchBox`/`SearchBoxRanked`, `Combo`, `StepGrid`, `PianoKeyboard` over a caller-owned `PianoKeyboardLayout`, `TransportScrubber`); display and status (`Avatar`, `Badge`/`BadgeDot`, `ColorIndicator`, `Icon`, `Text`, `Image`, `PageIndicator`); progress and loading (`RadialProgressBar` with `RadialCountdown`/`RadialCountUp`, `SkeletonLine`/`SkeletonRect`/`SkeletonCircle`); data and signals (`Histogram`, `HandleTrack`, `CurveTrack`, `DbMeter`, `GainReductionMeter`, `CorrelationMeter`/`StereoMetersState`, `Goniometer`, `Scope`, `Waveform` (zoomable over a caller-owned `TimelineView` and a `WaveformPeakSource`/`WaveformPeakCache`), `SpectrumAnalyzer` over a caller-owned `SpectrumAnalyzerState`, `EnvelopeEditor` over an `Envelope`, `ParametricEq` over `EqBand`s with `EqResponse` for the RBJ biquad response, and image and colour (`LevelsControl` over a caller-owned `LevelsAdjustment`, `AssetBrowser` over a caller-owned `AssetBrowserState`, whose `Apply` is the transfer function the widget draws, `GradientEditor` over a caller-owned list of `GradientStop`, with `SampleGradient` as the evaluator the bar `Toolbar`/`ToolbarSeparator` with `ToolbarButton`/`ToolbarToggleButton` (glyph and label in one frame, `ToolbarButtonLayout` picking leading, above-and-centred, glyph-only or label-only, the arithmetic in the ImGui-free `ToolbarGeometry`), is drawn with, `SwatchPalette`, `ImageCompare`); the `LogFrequencyAxis` both share); layout and containers (`DividerContainer`/`DividerZone`, `Grid`, `TabPanel`, `Card`, `Tree`, `ImageCanvas`, `PropertyGrid`, `OverlayHost`/`OverlayLayer`, `ScopedDisable`, `ScopedId`); and motion and gestures (`Tween`, `Spring`, `Easing`, `InertialScroll`, `GestureDetector`/`GestureMachine`). Also thin adapters delegating to `Hexa.NET.ImGui.Widgets`: `Spinner`, `BufferingBar`, `HorizontalSplitter`/`VerticalSplitter`, `ToggleSwitch`/`ToggleButton`/`TransparentButton`/`InlineButton`, `IconTreeNode`, `EnumCombo`, `TextCenteredV`/`TextCenteredH`/`TextCenteredVH`, `ImageCenteredV`/`ImageCenteredH`/`ImageCenteredVH`/`ImageScaleTo`, `Tooltip`, `Breadcrumb`, `DatePicker`/`YearPicker`, `FlameGraph`, `FileTreeView`, `OpenFileDialog`/`SaveFileDialog`/`OpenFolderDialog`, `RenameDialog`, `DialogMessageBox`/`ShowMessageBox`, `DockedWindow`. Seven of these look like duplicates of an existing ktsu widget; most are not, and the two that are have a recommended survivor — see [Hexa-backed vs ktsu widgets](#hexa-backed-vs-ktsu-widgets) below for the pair-by-pair verdict. `DatePicker` and `FileTreeView` need a Material Icons font registered via `FontHelper.AddCustomFont(io, data, size, FontHelper.GetMaterialIconRanges(), mergeWithPrevious: true)` (not `ImGuiAppConfig.Fonts`, which applies the Nerd Font mapping); see `examples/ImGuiAppDemo`. `YearPicker` needs no icon font. `OpenFileDialog`, `SaveFileDialog` and `OpenFolderDialog` need the same Material Icons font, for their toolbar, breadcrumb and file-tree glyphs; `RenameDialog`, `DialogMessageBox` and `ShowMessageBox` need none. `DockedWindow` composes Hexa's `ImWindow` internally rather than inheriting it — subclass it, override `Title` and `DrawContent()`, then call `Show()`/`Close()`. All of the dialogs and `DockedWindow` require a per-frame deferred-drawing pump; see [Deferred Drawing](#deferred-drawing-dialogs-and-docked-windows) below. Also includes callback-driven editors: `Sequencer`, `SequenceSource`, `CurveEditor`, `CurveSource`, `CurveData`, `BezierEditor`. Unlike the dialogs above, none of these need a deferred-drawing pump; see [Callback-driven editors](#callback-driven-editors) below.
+- **ImGui.Widgets** (`ktsu.ImGui.Widgets`) - Custom UI components, grouped as the README's feature list groups them: input and controls (`Switch`, `SegmentedControl`, `Stepper`, `TimecodeField` over the ImGui-free `Timecode`/`TimecodeRate`, `RangeSlider`, `ChannelFader`/`FaderTaper`, `XYPad`, `ColorWheel`/`LiftGammaGain`, `Knob`/`KnobWithDrag`, `Rating`, `Chip`/`ChipGroup`, `PinInput`, `SearchBox`/`SearchBoxRanked`, `Combo`, `StepGrid`, `PianoKeyboard` over a caller-owned `PianoKeyboardLayout`, `TransportScrubber`); display and status (`Avatar`, `Badge`/`BadgeDot`, `ColorIndicator`, `Icon`, `Text`, `Image`, `PageIndicator`); progress and loading (`RadialProgressBar` with `RadialCountdown`/`RadialCountUp`, `SkeletonLine`/`SkeletonRect`/`SkeletonCircle`); data and signals (`Histogram`, `HandleTrack`, `CurveTrack`, `DbMeter`, `GainReductionMeter`, `CorrelationMeter`/`StereoMetersState`, `Goniometer`, `Scope`, `Waveform` (zoomable over a caller-owned `TimelineView` and a `WaveformPeakSource`/`WaveformPeakCache`), `SpectrumAnalyzer` over a caller-owned `SpectrumAnalyzerState`, `EnvelopeEditor` over an `Envelope`, `ParametricEq` over `EqBand`s with `EqResponse` for the RBJ biquad response, and image and colour (`LevelsControl` over a caller-owned `LevelsAdjustment`, `AssetBrowser` over a caller-owned `AssetBrowserState`, whose `Apply` is the transfer function the widget draws, `GradientEditor` over a caller-owned list of `GradientStop`, with `SampleGradient` as the evaluator the bar `Toolbar`/`ToolbarSeparator` with `ToolbarButton`/`ToolbarToggleButton` (glyph and label in one frame, `ToolbarButtonLayout` picking leading, above-and-centred, glyph-only or label-only, the arithmetic in the ImGui-free `ToolbarGeometry`), is drawn with, `SwatchPalette`, `ImageCompare`, `CropOverlay` over an `ImageCanvas`); the `LogFrequencyAxis` both share); layout and containers (`DividerContainer`/`DividerZone`, `Grid`, `TabPanel`, `Card`, `Tree`, `ImageCanvas`, `PropertyGrid`, `OverlayHost`/`OverlayLayer`, `ScopedDisable`, `ScopedId`); and motion and gestures (`Tween`, `Spring`, `Easing`, `InertialScroll`, `GestureDetector`/`GestureMachine`). Also thin adapters delegating to `Hexa.NET.ImGui.Widgets`: `Spinner`, `BufferingBar`, `HorizontalSplitter`/`VerticalSplitter`, `ToggleSwitch`/`ToggleButton`/`TransparentButton`/`InlineButton`, `IconTreeNode`, `EnumCombo`, `TextCenteredV`/`TextCenteredH`/`TextCenteredVH`, `ImageCenteredV`/`ImageCenteredH`/`ImageCenteredVH`/`ImageScaleTo`, `Tooltip`, `Breadcrumb`, `DatePicker`/`YearPicker`, `FlameGraph`, `FileTreeView`, `OpenFileDialog`/`SaveFileDialog`/`OpenFolderDialog`, `RenameDialog`, `DialogMessageBox`/`ShowMessageBox`, `DockedWindow`. Seven of these look like duplicates of an existing ktsu widget; most are not, and the two that are have a recommended survivor — see [Hexa-backed vs ktsu widgets](#hexa-backed-vs-ktsu-widgets) below for the pair-by-pair verdict. `DatePicker` and `FileTreeView` need a Material Icons font registered via `FontHelper.AddCustomFont(io, data, size, FontHelper.GetMaterialIconRanges(), mergeWithPrevious: true)` (not `ImGuiAppConfig.Fonts`, which applies the Nerd Font mapping); see `examples/ImGuiAppDemo`. `YearPicker` needs no icon font. `OpenFileDialog`, `SaveFileDialog` and `OpenFolderDialog` need the same Material Icons font, for their toolbar, breadcrumb and file-tree glyphs; `RenameDialog`, `DialogMessageBox` and `ShowMessageBox` need none. `DockedWindow` composes Hexa's `ImWindow` internally rather than inheriting it — subclass it, override `Title` and `DrawContent()`, then call `Show()`/`Close()`. All of the dialogs and `DockedWindow` require a per-frame deferred-drawing pump; see [Deferred Drawing](#deferred-drawing-dialogs-and-docked-windows) below. Also includes callback-driven editors: `Sequencer`, `SequenceSource`, `CurveEditor`, `CurveSource`, `CurveData`, `BezierEditor`. Unlike the dialogs above, none of these need a deferred-drawing pump; see [Callback-driven editors](#callback-driven-editors) below.
 - **ImGui.Popups** (`ktsu.ImGui.Popups`) - Modal dialogs: MessageOK, Prompt, InputString/Int/Float, FilesystemBrowser, SearchableList
 - **ImGui.Color** (`ktsu.ImGui.Color`) - Bridge between `ktsu.Semantics.Color` and ImGui. Colors are held as the semantic `Color` (linear) and `Srgb` types and converted only at the ImGui seam: `ColorImGuiExtensions` (`ToImColor`/`FromImColor`, `ToImGuiVector4`, `ToImGuiU32`) and `SrgbImGuiExtensions` (`Srgb` → `ImColor`/`ImGuiVector4`/`ImU32`, packed directly with no linear round-trip). The `ImColor` and `Srgb` `ToImGuiU32` apply the global style alpha like `ImGui.GetColorU32`; the linear `Color.ToImGuiU32` is a pure pack matching `ColorConvertFloat4ToU32`. `ImColor` extension operations: adjustments (lighten/darken, saturate/desaturate, hue offset, grayscale, invert, alpha), analysis (relative luminance, contrast ratio, perceptual distance), and contrast heuristics (`MostReadableTextColor`, `AdjustForSufficientContrast`). All color math delegates to `ktsu.Semantics.Color`. (There is no `ImColor` factory class — construct via `Color`/`Srgb` and convert.)
 - **ImGui.Styler** (`ktsu.ImGui.Styler`) - Theming system with 50+ built-in themes, scoped styling, Button.Alignment, Text.Color semantic colors, Indent utilities, Alignment helpers, theme-aware color palette (`Palette`, e.g. `Palette.Basic.Red`, `Palette.Semantic.Error`), and interactive theme browser. Color construction and manipulation live in `ImGui.Color`.