Chart-family presentation presets: extend the single Style selector for Bar, Column, Area, and Pie

[BorisTyshkevich]

Problem

Line and Area have a useful Style control, while Bar, Column, and Pie need the
same compact preset UX without adding renderer types such as stacked-bar,
doughnut, or compact-column.

These variants share the result contract of their base chart type. They belong
in the existing Style selector and in the canonical panel.cfg.style object.

Goal

Keep the standard control row compact:

Type | Style | X | Y | Series

Presets update one type-specific object:

panel.cfg.style

Advanced combinations remain editable in Spec and appear as a disabled
Custom selection when they match no named preset.

Presets

Bar and Column

Bar remains horizontal and Column remains vertical. Both expose:

Preset	Mode	Density	Scale	Legend	Grid	Axes
Grouped	grouped	normal	zero	auto	auto	show
Stacked	stacked	normal	zero	auto	auto	show
Compact	grouped	compact	zero	auto	auto	show
Joined	grouped	joined	zero	auto	auto	show
Minimal	grouped	compact	zero	hide	hide	show
Data range	grouped	normal	data	auto	auto	show

{
  "type": "bar",
  "style": {
    "mode": "stacked",
    "density": "normal",
    "scale": "zero",
    "legend": "auto",
    "grid": "auto",
    "axes": "show"
  }
}

An explicit scale: "auto" uses the chart-family default: Bar and Column are
zero-based. scale: "data" opts into the data range.

Area

Preset	Curve	Points	Stack	Scale	Legend	Grid	Axes
Clean	linear	auto	overlay	data	auto	auto	show
Smooth	smooth	auto	overlay	data	auto	auto	show
Stepped	stepped	auto	overlay	data	auto	auto	show
Points	linear	show	overlay	data	auto	auto	show
Stacked	linear	auto	stacked	data	auto	auto	show
Zero-based	linear	auto	overlay	zero	auto	auto	show
Minimal	linear	hide	overlay	data	hide	hide	show
Sparkline	linear	hide	overlay	data	hide	hide	hide

Stacking is additive visualization only. Do not normalize values or add
percentage stacking.

Pie

Preset	Shape	Legend	Frame
Pie	pie	show	normal
Donut	donut	show	normal
Compact	donut	hide	compact

Donut maps to a fixed 60% cutout. Compact reduces layout padding, retains
tooltips, and is intended for small tiles.

Canonical type-specific Style branches

Line:
  curve, points, scale, legend, grid, axes

Area:
  curve, points, stack, scale, legend, grid, axes

Bar / Column:
  mode, density, scale, legend, grid, axes

Pie:
  shape, legend, frame

Unknown fields and future string values remain valid. Type switching preserves
the complete style object, including dormant fields, so switching back is
lossless. Each renderer reads only fields relevant to its current type.

There is no panel.cfg.display, Display selector, compatibility copy, or Spec
version bump.

Renderer mapping

• Grouped Bar/Column: both axes have stacked = false.
• Stacked Bar/Column: both axes have stacked = true.
• Compact density: categoryPercentage = 0.9, barPercentage = 0.95.
• Joined density: both percentages are 1, with borderRadius = 0.
• Stacked Area: one shared stack id and a stacked value axis.
• Pie: cutout = 0; Donut: cutout = 60%.
• Explicit scale: "auto": Line/Area use the data-range default; Bar/Column
  use the zero-based default.

Data semantics

This issue changes presentation only. It must not change X grouping,
duplicate-cell SUM aggregation, Series pivot, selected Y measures, row caps,
or SQL ownership of values. Joined spacing is not a Histogram contract.

Tests

Cover every exact preset and Custom state; grouped/stacked and density mapping;
Area overlay/stack without normalization; Pie/Donut/Compact; dormant-field and
unknown-extension preservation; schema/completion and share/Library round trips;
workbench/Dashboard parity; and direct explicit scale: "auto" rendering for
Line, Area, Bar, and Column.

Non-goals

• Display selector or cfg.display
• 100% stacking or per-dataset stack ids
• waterfall or Histogram semantics
• separate Doughnut type
• exploded slices, slice labels, or arbitrary renderer options

Acceptance criteria

• Bar and Column use one Style selector with six presets.
• Area uses one Style selector with eight presets.
• Pie uses one Style selector with Pie, Donut, and Compact.
• Presets update all owned presentation fields under cfg.style.
• Advanced unmatched combinations show Custom.
• Doughnut is a Pie style, not a new type.
• Type switching preserves complete dormant style data.
• Bar/Column explicit scale: "auto" remains zero-based.
• Existing data semantics remain unchanged.
• Schema and completion document the type-specific Style branches.
• Workbench and Dashboard render identically from the same Spec.
• No Display selector or cfg.display is added.
• npm test passes with the coverage gate.
• npm run build succeeds.
• No new runtime dependency is added.

Definition of done

Bar, Column, Area, and Pie gain their useful Grafana-like variants through the
same single Style control, while the Spec retains exact control over every
type-specific presentation field in one canonical object.