docs(charts): the recipe documents the styling surface it has

[DemchaAV]

Why

The chart recipe is the only page describing chart styling, and it had drifted from
the builder in three ways.

Five settings appeared nowhere on it — axisTextStyle, legendTextStyle,
valueLabelTextStyle, donutCenterTextStyle, barWidthRatio — though four are
load-bearing in the flagship examples that put a chart on a dark card. The only way
to find them was to read ChartStyle.Builder.

valueLabelHalo was documented under line charts, the one place its default white
chip is least likely to be wrong, rather than as the backing behind value and slice
labels alike. The single setting that must change when a chart leaves a white page
was filed where a bar or donut author would not look.

And ChartTheme was presented as a layer an author styles through. A chart placed
through the DSL resolves its geometry during the layout pass, after the document's
theme is out of reach (ChartDefinition.java:67), so it always starts from
ChartDefaults.DEFAULT_THEME and ChartStyle is the author-facing override. The
low-level ChartLayoutResolver.resolve(...) does take an explicit ChartTheme —
the page now draws that line rather than implying either that the theme is a knob
or that nothing accepts one.

What

• Every ChartStyle setting in one table with its default and what it applies to,
  marking which are inherited from ChartDefaults.DEFAULT_THEME and which are fixed
  engine defaults.
• A section for the halo of its own: what it is for, why white is wrong off a white
  page, and translucency through withOpacity(...).
• A typography section for the three chrome text styles; donutCenterTextStyle
  documented beside the centerText it styles; barWidthRatio in the bar section.
• ChartStyleDocumentationGuardTest reflects over ChartStyle.Builder and fails the
  build when a setter is not named on the page.

Tests

• ./mvnw -B -ntp clean verify → BUILD SUCCESS.
• The guard was confirmed able to fail: with barWidthRatio and donutCenterTextStyle
  removed from the page it reported exactly those two.
• The three new snippets carry doc-example markers, so DocumentationSnippetCompileTest
  compiles them against the current API; breaking valueLabelHalo produced a compiler
  error attributed to charts-halo.

Base

Rebased onto develop at #501, which is what makes the halo section true for every
chart kind: grouped-bar value labels passed null until that fix, so the page would
otherwise have promised behaviour the default bar chart did not have.

[DemchaAV]

All three are real, and the branch is now rebased on develop at #501, so the halo section is unconditionally true — both BarChartLayout call sites pass halo.

1. ChartTheme. You are right and my claim was simply too wide. ChartLayoutResolver is a public final class, resolve(...) is public static with no @Internal, it takes a ChartTheme, and ChartTextMetrics is a public interface — so the seam is genuinely callable, not theoretical. The page now separates the authoring path (always ChartDefaults.DEFAULT_THEME, ChartStyle is the override) from the geometry seam that does accept a theme; CHANGELOG says "no authoring API accepts one".

This one had a second half neither of us named: the layers table at the top called ChartLayoutResolver an "internal pure function (data, spec, style) → primitives" — internal for something public, and a signature that does not exist (the real one is (spec, style, theme, width, height, metrics)). Fixed in the same pass, since it is the same error one layer up.

2. Footnote. Confirmed: lineWidth and valueLabelOffset live in package-private ChartLayoutSupport, areaOpacity is private in LineChartLayout, sliceGapDegrees is a literal in PieChartLayout, and pointMarker / square corners are the absence of an override rather than any constant. Reworded to your form, with the nuance kept that three unmarked rows (barWidthRatio, sliceStroke, donutCenterTextStyle) are ChartDefaults constants.

3. valueLabelOffset on pie/donut. Confirmed at PieChartLayout:61 and :114 — valueLabelGap(style) sets the radial gap between the rim and the slice label. Fixed in both the table row and the prose.

Worth noting where the guard's limit is: it checks that each setter is named on the page, so it would not have caught any of these three. They are claims about behaviour, not missing names — that part still rests on review.

clean verify → BUILD SUCCESS. Amended rather than stacked a fix commit, so the branch is still one commit.