v2.4.0 — Every flowchart link, every classic shape

Mermaid's full set of flowchart link styles and its classic node shapes now
draw as written, where 2.3.0 drew them as plain arrows, circles and
rectangles. Rendering only changes for diagrams that use this syntax.

Source compatibility. Flowchart.NodeShape gains six cases. Code
outside the package that switches exhaustively over NodeShape stops
compiling until it handles them; an @unknown default: branch turns that into
a warning instead. This follows the 1.2.0 precedent (hexagon, subroutine). Everything else is
additive: the new Edge and PlacedEdge fields have defaults, and the
wire format and the Android, Flutter and Windows backends are unchanged.

• Link styles. Thick links (==>, ===) draw at 2pt, and invisible
  links (~~~) are laid out but not drawn: they rank and order nodes but
  never move a drawn edge or label, and the linter, ASCII renderer and alt
  text skip them. Circle (--o) and cross (--x) heads work at one end or
  both, on any stroke and with inline text. Before, they were plain arrows.
  New fields: Flowchart.Edge.head, .thick, .invisible, .length, and
  PlacedEdge.head, .thick, .invisible. (#68)
• Link length. Each extra -, =, . or ~ adds a rank, counted as
  mermaid counts them (---> spans two). It's capped at 10 per link and 100
  extra ranks per chart, so a pathological chart still lays out in well under
  a second. (#68)
• Node shapes. doubleCircle ((( ))), parallelogram [/ /],
  parallelogramAlt [\ \], trapezoid [/ \], trapezoidAlt [\ /] and
  asymmetric > ], plus their @{ shape: … } names (dbl-circ,
  lean-r/in-out, lean-l/out-in, trap-b/priority, trap-t/manual,
  odd). Edges land on the slanted sides and the flag's notch, not on the
  frame. (#68)
• DOT and ASCII. doublecircle, parallelogram, trapezium and
  invtrapezium map both ways. style=bold/invis, dot-family arrowheads and
  minlen carry thick, invisible, circle heads and length. arrowhead=none /
  arrowtail=none now suppress that end. The terminal renderer has glyphs
  for the new shapes. (#68)

Thanks to @hipstersmoothie, who wrote this from the diagrams coding agents
actually emit.

Fixed

• The docs gallery flowchart draws Scene IR as the parallelogram it's
  written as. (#68)

Docs

• A full documentation audit, every claim checked against the code: the
  README, website, DocC catalogs, API doc comments, platform READMEs and design
  notes. Highlights: the website's lint demo used rule names that don't exist
  and a mermaidkit-lint command that was never built; conformance claims now
  say CI gates macOS, Linux, WASM and Windows (Android was verified on an
  emulator); the terminal CLI's tier order and flowchart-only box tiers are
  described as they behave; DOT round-trips list their lossy cases; the
  coverage audit and dated design memos carry v2.4.0 reconciliation notes; the
  contributor guide covers the RenderScene lowering and the semver rule as
  practiced. Both DocC catalogs now build with zero warnings (nine broken
  symbol links fixed), and the README has an install snippet.