Follow figure's main branch: the charts it grew, and a fallback font

[timzifer]

The branch started as one horizon chart and has followed figure's main branch
since. It is now the demo's catalogue caught up with what figure can draw,
plus the one widget change that catching up asked for.

The widget change

figure's backend/gg grew WithFallbackFont: fonts consulted, in order, for
a rune the chart's own font has no glyph for, with a ? written for a rune no
face can draw at all. That closes the gap chart.ThemeFont has carried a
warning about since it was written — Fyne's NotoSans-Regular has no ≤, ≥
or ∞, and a rasterizer handed one font drew nothing where the shaper Fyne
draws its own text with would have fallen back.

• fynefigure.FallbackFont is the new option, and Target.SetFont takes the
  fallbacks with the typeface, so a chart that follows the application to a new
  font keeps them. The config holds the faces rather than the rasterizer's
  options: SetFont used to rebuild that list, and would now have dropped the
  fallback while replacing the font.
• The chart widget passes look.Fallback() — the Go regular face, which is
  what every other figure raster is drawn in — whenever it follows the theme's
  typeface, and the tooltip beside it gets the same list.
• The test draws the same title as "????" and requires the two to differ. A
  question mark is ink, so counting ink no longer tells a drawn symbol from a
  lost one.

The charts

Each entry is a chart type figure grew, because the demo is where one has to
appear or nobody sees it.

• horizon — a series folded into bands of equal height, named by a
  colourbar instead of an axis, with the band height pinned rather than cut
  from the data
• loci, and the tree opening on what is new
• bars-extruded, weibull, survival, roc, correlogram, spectrogram,
  dendrogram-heatmap, radial-tree, spc — the raster mark, the probability
  scales, the statistical instruments and the tidy tree
• smith-sphere, antenna-pattern, bloch, scatter3 — the spherical scene as
  three charts plus the 3D scatter
• soil-texture, alloy-prism — the barycentric coord and the ternary prism
  over it, in a new Ternary group beside Polar and Smith. The prism is
  sampled over the whole simplex, so the liquidus is a surface rather than the
  corner a real alloy catalogue would crowd into.
• ribbon — a descending approach as a band, which occludes and says which
  of two passes is in front
• joint-orientations — two fracture sets over a scattered background,
  binned into cells of equal area: the chart a stereonet's contoured density is
  the flat form of

Three existing entries moved rather than being added to:

• bloch draws two dozen states rather than two hundred and forty, which is
  where Frame.Arc shows: a coarse sweep joined by chords would cut through
  the ball.
• dendrogram-heatmap puts the gene tree in a left track —
  geom.Orient(geom.Horizontal) with geom.Baseline(1) — so ADR 0053's
  headline chart is drawn whole.
• bars-extruded names a fill and a colour, which outlines all three faces
  of an extruded mark.

cmd/demo/plots/data.go's clustering is now a function of the items and a
distance, because the genes are clustered by the same code read the other way
round, and its uniform deviate takes three rounds: two read side by side off
one affine round walk in step, which put the histogram's background on a curve
instead of over the ball.

Base branch

Targets shared-selection, not main: cmd/demo/plots/ does not exist on
main, so this work sits on top of that branch rather than beside it.

CI

Green, and it took three fixes that had nothing to do with the charts.

• The requires named tags without the API the code calls. CI builds each
  module from its own go.mod, with no go.work, so the comment saying "builds
  through go.work until figure is released" was a red build. All three modules
  now name a commit of figure's main branch — the widget calls
  gg.WithFallbackFont, the demo draws the chart types after v0.11.0, and the
  GPU tier calls Available, Disable and Enable, which backend/gg/gpu has
  never released. Each go.mod says why it is not a tag and when it goes back
  to being one.
• The Linux binary still carried the GPU tier. Its import moved behind
  !linux a while ago, but three files went on importing it for Enabled,
  Available, Enable, Disable and Close, so the link still ended in
  dlopen_stub: unhandled relocation for goffi_dlopen. CI never said so because
  module resolution failed first. The tier is now reached through six functions,
  the import lives in the one file Linux does not build, and tier_linux.go
  answers instead. GOOS=linux go list -deps went from eight matches on the
  tier and its foreign-function layer to none.
• The demo's tests raced. The kitchen refreshes the panel from a ticker
  through fyne.Do; the test driver documents that it runs the function on
  whichever goroutine called it, so the ticker was a second goroutine in the
  widgets the test was driving. newKitchen takes the interval and the test
  passes zero.

Test plan

• everything below was run with GOWORK=off, which is how CI builds it
• root module: go build ./... && go vet ./... && go test ./... — passes,
  including the new font test and its negative control (with the fallback list
  emptied, the symbol titles render identically to "????" and the test fails)
• cmd/demo: go build ./... && go vet ./... && go test ./... — passes;
  TestEveryEntryBuildsAndPaints paints every new entry in a Fyne test window,
  and TestCatalogueShape checks the new group's order and ids
• gpu module: go build ./... && go vet ./...
• gofmt -l . — clean in both modules
• every new and changed entry was rendered to PNG and looked at

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