scale, coord, render: axis breaks and folds, marked or not drawn

[timzifer]

Summary

A linear or time axis can now leave intervals out, and the gap is always marked. See ADR 0083.

• Breaks: scale.Break(lo, hi) and scale.TimeBreak(from, to). Use one for a chart where a single outlier would flatten every other bar.
• Folds: scale.Fold(ivs...) and scale.TimeFold(spans...). Use them for many stretches the reader does not want to look at, such as a machine's idle periods. A fold leaves a narrow gap and gets a small slash, never a zigzag.
• Table helpers: figure.SpansWhere(tbl, "start", "end", "state", "Inaktiv") reads the folds out of a table. SpansFunc, IntervalsWhere and IntervalsFunc cover other tests and numeric axes.
• Theme: new fields AxisBreakGap, AxisBreakSize, AxisBreakMark, AxisFoldGap and AxisFoldSize, set through theme.AxisBreaks and theme.AxisFolds. BreakSlash (//) is the default and BreakZigzag is the alternative.
• Dialect: a scale writes "cuts" and "folds". "breaks" is already the colour scale's class boundaries.

How it works

• Map stays continuous. A value inside a cut maps across the gap and the panel's clip hides it. Invert stays exact, and a bar across the break is still one bar with its middle missing.
• Render sets the gap. It does this once per render through scale.Breaker. Under any coord other than Cartesian, and on second axes, render switches breaks off, because a break without its mark lies about distances.
• The Cartesian coord draws the break. It cuts the clip into one rectangle per kept piece and splits the axis and grid lines at the gaps. The marks are drawn after the data, so a bar crossing the break cannot cover them.
• A cut at an end of the domain trims the axis instead of leaving a gap with nothing on its far side.
• Ticks use one step for the length the axis still shows, e.g. 0, 5, 10 | 90, 95, 100.
• Lookup is a binary search, so a fold costs nothing per row. BenchmarkFolded1k and BenchmarkFolded100k both come out at 102 allocs and are now in the allocation gate.
• The a11y description names the omitted intervals.

What does not change

• A chart without breaks draws exactly what it drew. Every existing golden file matches, and gallery -check reports the documentation figures up to date.
• New gallery plates: axis-break, axis-break-zigzag and machine-folds.
• New goldens: axis-break-slash, axis-break-zigzag, axis-break-time and axis-folds.

Known cost

A broken chart's clip is several rectangles, so gg masks it instead of scissoring it. Only charts with a break pay this.

Test plan

• go test ./... in the core and in backend/gg, including the gallery tests
• -race on scale, coord, render and the root package; a parallel facet with a break is byte-identical to the serial one
• The core's dependency check prints nothing
• The allocation gate is flat for BenchmarkFolded1k against BenchmarkFolded100k

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