feat: Add lline chart color selector - #708

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/mosaic/src/charts/chart-types/histogram/HistogramSettings.tsx`:
- Line 24: The code uses a non-null assertion on CHART_COLORS[0] (seen in
HistogramSettings.tsx value={config.settings.color ?? CHART_COLORS[0]!}) which
can crash if CHART_COLORS is empty; update places that use CHART_COLORS[0]
(including spec.ts) to defensively handle an empty palette by either using
nullish coalescing to a hard-coded fallback color (e.g., config.settings.color
?? CHART_COLORS[0] ?? '`#000000`') or add an explicit validation early (validate
CHART_COLORS.length > 0 and throw a clear error) so the UI and module loading
cannot fail silently.

In `@packages/mosaic/src/charts/chart-types/line-chart/spec.ts`:
- Line 23: The legend string uses locale-sensitive casing; replace the call to
toLocaleUpperCase() on yColumn.aggregate with the locale-invariant toUpperCase()
so the legend for aggregate tokens (yColumn.aggregate) is deterministic across
locales—update the expression that currently returns `${yColumn.field}
(${yColumn.aggregate.toLocaleUpperCase()})` to use toUpperCase() instead.

In `@packages/mosaic/src/components/ColorSelector.tsx`:
- Around line 27-33: The color swatches in ColorSelector.tsx (inside the
CHART_COLORS.map and Combobox.Item) are purely decorative and not announced to
screen readers; update each Combobox.Item to include an accessible label
containing the color value (e.g., the hex string from CHART_COLORS) by either
adding an aria-label on the Combobox.Item (aria-label={color}) or by rendering a
visually-hidden text node (span with sr-only class or role="status" as
appropriate) next to the swatch so non-visual users can identify the color
choice; keep the visual swatch markup the same but ensure the Combobox.Item
exposes the hex value for assistive tech.

In `@packages/mosaic/src/components/MultiFieldSelector.tsx`:
- Line 12: MultiFieldSelector currently hardcodes line-chart behavior by
importing getLineColor and defaulting to the 'sum' aggregation inside its
internal item creation; make it chart-agnostic by removing the getLineColor
import and the 'sum' default and instead accept a prop like
createFieldConfig(fieldName: string, index: number) => FieldConfig (or similar)
that returns the initial config for each item. Update MultiFieldSelector to call
props.createFieldConfig when adding or initializing items (replace uses of
getLineColor and the 'sum' literal), provide a sensible fallback no-op factory
if none is passed, and document the new prop so callers (e.g., the line-chart)
can inject their color/aggregation defaults.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In `@packages/mosaic/src/components/MultiFieldSelector.tsx`:
- Around line 11-25: The component is typed to the line-chart-specific
YFieldConfig; change MultiFieldSelectorProps to be generic so the component is
reusable: replace YFieldConfig usage in the public API (the
MultiFieldSelectorProps interface, value, onChange, onAdd, and renderItem
signatures) with a generic type parameter T (e.g., T extends { field: string }
or a small shared BaseField type defined in the components package) and update
renderItem and handleUpdate to use Partial<T>; remove the direct import of
YFieldConfig and, if needed, have the line-chart package pass its YFieldConfig
as the concrete T when using the component.

---

Nitpick comments:
In `@packages/mosaic/src/charts/chart-types/histogram/HistogramSettings.tsx`:
- Line 9: getChartItemColor is a generic color utility currently imported from a
line-chart-specific module (line-chart/utils) but used by multiple chart types;
move the function to a shared module (e.g., constants/chart-colors.ts or
utils/chart-utils.ts), export it there, update imports in HistogramSettings.tsx
and other callers to import getChartItemColor from the new shared path, and
remove the original export from line-chart/utils to avoid duplicate exports;
ensure the relocated function keeps its original name and behavior and update
any relevant docs/comments.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/mosaic/src/charts/chart-types/line-chart/utils.ts`:
- Around line 1-11: Add a JSDoc block to the public function getChartItemColor
describing its behavior (returns the provided color or a fallback from the
colors array based on index), document parameters (colors: readonly string[] -
must be non-empty, color: string | undefined, index: number default 0), the
return type (string), and any preconditions/notes (colors should contain at
least one entry; index is modulo-length). Include `@param` and `@returns` tags and
mark it as part of the public API (e.g., `@public`) so callers know expected
inputs and behavior.
- Around line 1-11: The getChartItemColor function currently assumes the colors
array is non-empty; validate that colors.length > 0 (or explicitly document the
precondition) and handle the empty case to guarantee a string return. Update
getChartItemColor to check colors is non-empty and either throw a clear Error,
return a safe default color string, or fallback to the provided color parameter
when colors is empty, ensuring the function never returns undefined; reference
the function name getChartItemColor and the parameters colors, color, and index
to locate where to add the guard and error/default behavior.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/mosaic/src/charts/chart-types/line-chart/utils.ts`:
- Around line 6-21: getUnusedColor assumes the input array colors is non-empty
which can lead to returning undefined; add a guard at the start of
getUnusedColor that validates colors.length > 0 and throws a clear Error (e.g.
"getUnusedColor: colors must contain at least one color") if empty, and update
the function JSDoc to document the precondition that colors must be non-empty;
keep the existing logic (usedSet loop and fallback to colors[0]) intact after
the validation.

In `@packages/mosaic/src/constants/chart-colors.ts`:
- Line 5: DEFAULT_CHART_FALLBACK_COLOR currently uses a non-null assertion on
DEFAULT_CHART_COLORS[0], which can break if categoricalSchemeColors.Observable10
is empty; update the initialization of DEFAULT_CHART_FALLBACK_COLOR to
defensively handle an empty array by checking DEFAULT_CHART_COLORS (or the
source categoricalSchemeColors.Observable10) and either (a) use a hard-coded
safe color string as a fallback via nullish coalescing, or (b) validate and
throw a clear error at module load if the palette is invalid; refer to
DEFAULT_CHART_FALLBACK_COLOR and DEFAULT_CHART_COLORS when making the change so
consumers always receive a valid color or a clear failure message.