0.6.0 — egui 0.36 / wgpu 30, deferred GPU sync, form helpers

Changed

• egui / egui-wgpu 0.36 (wgpu 30), MSRV 1.95. rsplot re-exports
  egui, so downstreams move to 0.36 with it — hence the minor bump. Adapted:
  wgpu 30's Option vertex buffers and fallible get_mapped_range, egui's
  Panel::show_inside → show, egui_kittest::create_render_state taking
  RendererOptions; the headless widget tests run through
  test_util::HeadlessRun, since epaint 0.36 panics when an unapplied
  TexturesDelta is dropped.
• rsplot: WgpuBackend defers sync_gpu_items to one flush_scene per
  frame, so N item mutations (add / remove / update, z, visibility, clear)
  rebuild the GPU scene once instead of N times; unchanged z / visibility
  writes no longer dirty the scene.
• rsplot: the title and the x, y, y2 and extra-axis labels lay out
  single-line and elide with … to their gutter run instead of being clipped
  mid-glyph on a narrow plot; overlapping tick labels are culled on every
  gutter (tick marks never).
• rsplot: the compact-toolbar overflow button uses U+2026, which egui's
  bundled font covers (U+22EF rendered as the tofu box).

Added

• rsplot: widget::form — label / fill / group helpers for Qt-style
  (QFormLayout) dialog rows in one egui pass; the colormap, print and
  limits dialogs use them.
• rsplot: a whole-widget plot background: set_background_colors' first
  argument was dropped, so a light canvas over a dark host panel got dark
  tick labels on still-dark gutters. When set, PlotView paints the widget
  rect with it under the data layer.
• Documented that hosts must append a broad-coverage fallback font for the
  arrow and box-drawing glyphs the ROI descriptions, the style selector and
  the image-compare toolbar use.

Fixed

• rsplot: a range ROI's name label stays inside the data area instead of
  landing in the title gutter over the plot title.

Full changelog: v0.5.6...v0.6.0