keynot 0.3.0

Added

• Background gradients: colors.background accepts
  { gradient: ['#hex', ...], direction: vertical | horizontal | radial }
  in addition to a solid color. The dark theme's default background is
  now a slate gradient (#2d2d30 down to #181818).
• Code blocks render as small terminal windows: a rounded border with
  traffic-light dots, the language name in the bottom edge, and a panel
  darker than any background. code_style: plain restores the bare
  panel; the frame color is themeable as colors.code_border.
• The speaker line highlight is column-aware: down/up walk lines within
  a column, left/right move between columns, and moving past the
  slide's edge changes slides (so single-column decks behave exactly as
  before).
• keynot check errors on unknown frontmatter keys, listing each with
  its line number and the valid keys; keynot play ignores them so a
  deck written for a newer keynot still opens on an older one.
• Error reports are colorized on a terminal (respecting NO_COLOR);
  piped output stays plain.
• FORMAT.md documents every frontmatter default, enforced by tests that
  fail when the docs drift from the code.

Changed

• The dark theme's code panel darkened to #141414 so it stands out
  against the gradient background.

Fixed

• Reloading with r no longer freezes the show while URL images
  re-fetch; decodes happen on a worker thread and the network is never
  touched from the draw path.
• A highlight begun during a slide transition no longer lands on an
  arbitrary line of the incoming slide.