\section{Third-party libraries and backend providers}
The independent starter lives in \key{docs/libraries/starter/}. It contains a
public-only library, an example, a three-engine checker and compatibility guidance.
\begin{source}
\usepackage[libraries=none]{gradbars}
\gradbarsdeclarelibrary{starter}{table}{gradbars-contrib-starter.sty}
\usegradbarslibrary{starter}
\end{source}
Keep the contributed file on the TeX search path. Rename starter package/type/
formatter/style/backend names when making your own library. Declare dependencies
in the registry and check them in the library. \key{\gradbarsversion} reports the
package version; \key{\gradbarsrequireversion{1.1.0}} checks a minimum semantic version.
Run the standalone checker with texlua and the installed TeX engines:
\begin{source}
texlua docs/libraries/starter/check-starter.lua --gradbars-root .
\end{source}
\subsection{Public scalar backend contract}
\key{\gradbarsbackendversion} is schema 1. It provides a scoped read-only record
for bar and Heat callbacks while keeping private model storage private.
\begin{source}
\gradbarsdeclarebackend{my-backend}
 {bar,heat,missing,signed,gradient,print,labels,units,clip,
  decorations,embedded,tagging,artifact}
 {\gradbarsbackenddefault}
\gradbar[backend=my-backend]{72.125}
\end{source}
The callback receives bar or heat as its first argument. Declare only implemented
or delegated capabilities. \key{\gradbarsbackendvalue{field}} exposes canonical
values, normalized positions, physical dimensions, options and descriptions during
the callback. \key{\gradbarsbackendlabel} typesets the existing formatted label.
\key{\gradbarsbackenddefault} draws the captured native continuation once.
\key{\gradbarsifbackendcapable{name}{feature}{yes}{no}} queries a declaration.
Unknown backends, missing capabilities, recursive callbacks and out-of-scope
record reads are diagnosed. Unsupported compound/legend kinds require explicit
\key{backend fallback=tikz}; default error prevents silent changes. Default
rendering still uses TikZ. This contract does not supply a second renderer.
Callbacks may run during discarded table measurements: avoid external writes,
respect suspended tagging and preserve raw data, units, clipping and baselines.
The complete field and capability contract is in \key{docs/backends.md}.