fix(charts): grouped bars emanate from the zero baseline

[DemchaAV]

Why

Grouped (non-stacked) bar charts measured each bar's height from the axis
nice-floor rather than zero. On an axis that crosses zero this rendered a
negative value as a short upward column anchored at the floor — visually
indistinguishable from a small positive — and let positive bars overshoot
below zero. Stacked bars were already pinned to zero, so the two groupings
disagreed.

What changed

BarChartLayout now anchors both orientations to the zero baseline, clamped
into the visible range by the new baselineValue(NiceScale) helper
(max(niceMin, min(0, niceMax))):

• Positive bars grow away from zero; negative bars hang back across it; the two
  meet exactly at the zero line (heights proportional to |value|).
• When zero is off-scale — an explicit non-zero valueAxis().min(...) or
  baselineAtZero(false) over a range that excludes zero — the baseline clamps
  to the nearest visible bound, so a deliberately zoomed axis still anchors its
  bars at the plot floor (explicitAxisBoundsAreHonored keeps passing).
• Value labels follow the bar's far end: above/right for positive, below/left
  for negative.
• The stacked path is untouched. Charts with positive data on a zero-based axis
  are byte-identical.

ChartShowcaseExample gains a mixed +/- net-cash-flow card; the committed
assets/readme/chart-showcase.{pdf,png} previews are regenerated.

Verification

• ./mvnw test -pl . → BUILD SUCCESS, 1384 tests, 0 failures.
• ChartLayoutResolverTest: rewrote the negative-grouped case to assert bars
  meet at zero with heights proportional to |value|; added horizontal,
  positive-min-clamp, and OUTSIDE-label-side cases. Mutation-checked — copying
  either negative value-label arm into its positive counterpart turns the new
  label tests red.

Lane: canonical (document.chart). No public API change (baselineValue is
package-private); japicmp unaffected.