Add StepGrid step-sequencer widget

[matt-edmondson]

Requested by Matt · project thread

Before: ktsu.ImGui.Widgets has no step sequencer. A drum pattern or a gate sequence means drawing a grid of cells and writing your own click and drag handling.

After: ImGuiWidgets.StepGrid(label, steps, rows, stepCount, playingStep, stepsPerBeat, cellSize, rowLabels) draws a rows × steps grid over a caller-owned Span<bool>. Pressing a cell toggles it, and dragging from there paints the same value across every cell the pointer crosses. It shades beat groups, highlights the playing column, and draws optional row labels. The method returns true on any frame the pattern changed. Each cell is probe-addressable as {label}/r{row}s{step}.

How: geometry, hit testing and the toggle-then-paint gesture live in StepGridLayout and StepGridState, which are internal and nested in ImGuiWidgets the same way HandleTrackState is. They need no ImGui context, and tests/ImGui.Widgets.Tests/StepGridStateTests.cs covers them (21 tests). The only item the widget submits is one InvisibleButton over the cells. It does not reuse Grid, as decided on the issue. tests/ImGui.Widgets.UITests/StepGridTests.cs (11 tests) drives the widget in isolation.

Also in this PR:

• A 4 × 16 drum-pattern demo with its own play clock, in examples/ImGuiWidgetsDemo/StepGridDemo.cs. For now it's registered with one line each in SignalDemos; that moves to DemoSections.cs once Split the widgets demo into one class per section #534 lands.
• A demo UI test that clicks a cell.
• A gallery tile.
• README and CLAUDE.md entries.

Deviations from the issue text worth checking:

• The drag segment is clipped to the grid before it is sampled. ImGui reports an unavailable mouse at -FLT_MAX, and sampling that unclipped segment would never finish. There are tests for leaving the grid and coming back, and for a -FLT_MAX or NaN pointer.
• The row-label tooltip uses BeginTooltip/TextUnformatted, not SetTooltip, so a label containing % isn't read as a format string.
• An empty grid (rows or stepCount ≤ 0) returns false before the span-length check, so it never throws.

Not included: the rendered tile image docs/gallery/widgets/stepgrid.png. This environment can't upload Git LFS objects, so it needs a local dotnet run -c Release --project tools/WidgetGallery -- --only StepGrid. The composite gallery sheets were not regenerated either.

Checks run locally in Release:

• Unit 21/21, widget UI 11/11, demo UI 5/5, WidgetGallery --check clean, gallery UI tests 5/5.
• 0 warnings across the demo and test projects.
• EverySection_CanBeExpandedWithoutError passes only once ktsu.png has been pulled from LFS. With a pointer checkout it fails at "Mobile - Decorators" on main too.

This branch appends to the same shared lists as the other widget PRs (#539 and the two to come): SignalDemos, WidgetDemoSections, InputAndControls.cs, and the CLAUDE.md and README feature lists. Any conflicts between them are adjacent additions, so resolve them by keeping both sides.

Closes #526

🤖 Generated with Claude Code

https://claude.ai/code/session_01PndoA4WHoRsyywzECYDNX3

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Generated by Claude Code

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Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.9% Coverage on New Code
0.0% Duplication on New Code

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