0.41.1 — tick, menu and gallery fixes

Documentation

• README and docs homepage corrected and tidied. Both still described the
  toolbar as it stood before the Slice tool and options= existed; both are now
  documented. Several claims had drifted and are fixed: 24 built-in colormaps
  (the list beside it already named 27), "a hundred-odd" real-application
  figures (there are over 160), "one font-metric family" (four are bundled), and
  a homepage claim that 3-D axes project onto the 2-D core -- there is no 3-D at
  all; polar is the thing that projects. barbs was listed as upcoming though it
  ships, and the plot-types table was missing it along with ecdfplot,
  kdeplot and pcolormesh_frames. "Supported plot types" is rewritten from a
  single run-on paragraph into one table per grouping, the Roadmap section is
  gone, and a new GIF shows the Slice companion panel driven by one coupled
  slider across two panels.

Changed

• Gallery figures no longer open with Slice switched on. Every tool is still
  offered in the toolbar; none starts active. Slice reshapes the axes it runs on
  (carving out a companion strip, docking a slider), so starting it enabled meant
  a reader met a modified figure before choosing to modify it. The five examples
  that specifically demonstrate Slice still turn it on themselves.

Fixed

• A toolbar menu could open past the edge of an embedded figure. A figure
  embedded in a page -- a Report panel, a docs gallery iframe -- gets whatever
  height the host gave it, and the Slice menu is the tallest of them. With
  nothing bounding it, the bottom of the list fell outside the document: clipped
  away rather than scrolled off, since an iframe has no viewport of its own to
  scroll, leaving those options unreachable. An open menu is now measured and
  kept inside the viewport -- scrolling internally, or flipping above its button
  when that side has more room -- and the same clamp keeps a menu near the right
  edge on screen.

• Tick decades are computed from the decimal literal, not pow. Neither
  Python's ** nor JavaScript's Math.pow is required to return the correctly
  rounded power of ten, and both miss: 10.0**23 and 10.0**126 differ from
  their literals, and some JavaScript engines return a different double for
  10**-5 than others. On an engine that rounds it the other way, an axis over
  [1, 1.0001] lost its first tick entirely once zoomed -- ceil(vmin / step)
  landed on the next integer -- so the interactive view disagreed with the ticks
  the static render had already drawn. Both sides now build a decade by parsing
  the decimal literal, which is correctly rounded by spec in both languages.