[hdcodedev]

Summary

• Every chart takes an optional subtitle. It shows below the title with a 4 dp gap and stays visible when a selection changes the title. A subtitle also works without a title. A null or blank subtitle takes no space.
• ChartContainerDefaults.style() takes styleTitle and styleSubtitle, and ChartContainerStyle has styleSubtitle. The subtitle defaults to 14 sp, normal weight, in the theme's onSurfaceVariant color (StyleDefaults.subtitle*).
• A new internal ChartHeading draws the title and subtitle for every chart, so spacing and visibility live in one place.
• Pie's selected title is now Category: share in one text, like the other one-series charts. It used to be the category followed by a smaller share in a separate text.
• Charts in a parent with unbounded width, such as a horizontalScroll row, keep their title and subtitle visible. Line, live line, gauge, pie, and radar share a new FiniteWidthChartSizeModifier for this. The bar family already sized its root this way. Making the two families size the same way in a verticalScroll is tracked in [Issue] Size all charts the same way when the parent leaves height unbounded #780.
• Docs: customization.md gains a Title and Subtitle section. pie-chart.md and the legend internals describe the new pie title.
• Screenshots: new ChartSubtitleScreenshotTest (line, selected pie, subtitle-only gauge). PieChartSelectedSlicePreview and the README showcase references change with the pie title.

Local Validation

• ./gradlew ktlintCheck
• ./gradlew :charts-<module>:jvmTest
• ./gradlew chartsTestJvm
• ./gradlew ciCompile
• ./gradlew chartsCheck
• ./gradlew apiCompatibilityCheck
• ../charts-playground/gradlew -p ../charts-playground -DchartsLocalPath="$PWD" ciCompile
• ./gradlew :androidApp:validateDebugScreenshotTest
• ./gradlew updateScreenshots