`bindings` mapped an object id to a single variable name, so binding an object
to a second variable silently dropped the first — a binding lost with nothing
to show for it. The reference app binds the selected object to the selected
variable, one click at a time, and nowhere says a second binding replaces the
first (https://helpx.adobe.com/illustrator/desktop/automate-visualize-data/
automate-actions/work-with-variables.html), so this allows several instead.
`bindings` is now a set of variable names per object. `dataset.select` walks
them in the order the variables are defined, which keeps two text variables on
one object deterministic (the later one's value is what is left, and `applied`
counts both). `variable.bind` reports `bound` (added) and `already` (was there
already); `variable.unbind` takes an optional `variable` to drop one binding
instead of all of them.
This is done before the field ships rather than after: `variables` has never
been in a released file, so the shape can still change for free. Once a version
with it is out, the same change would need a format version bump and a
migration, since `crates/format` keeps every version from 1 loading.
A strict superset of what worked before: every binding a document had is still
there and still applied, in the same order.
cargo xtask ci: all 7 steps pass.
eb0f2ff2e4