feat: Add the primitives a review page reads in - #1

The apps consuming this package had each rebuilt a labelled field, a
text box, a picker, a segmented switcher and a row in a list of things,
by hand and with slightly different spacing every time. All five now
come from here, so a change to how a form or a list looks is one change
rather than one per app.

A detail page is the same header with a back button and a badge beside the
name. Both apps were about to hand-roll a second header rather than reach for
this one, which is how two headers that look almost alike start.

A select, an input and a button standing in the same row were three
different heights, because each was written out by hand with whatever
padding looked right on its own. They share one scale now, at the heights a
button already came in, so a row of controls is one height and a caller who
wants the compact one asks for it by name.

Rules, and most of what people write down, are markdown documents rather
than a line of text. Every surface that had one was showing it as a block of
unformatted characters.

The colours come from the design tokens rather than the typography plugin's
own, so a document reads the same in either theme without a second palette to
keep in step. Markup in the source is not rendered: what goes through here is
written by whoever is using the product.

A box of lines is a text editor pretending to be a list: nothing says an
entry is malformed, nothing stops two of the same, and removing one means
selecting exactly the right characters. Folders, globs and tags are all
lists, and all of them were being typed into a box.

The brand colour here is a red, a few degrees from the one that means
something failed. Every screen that wanted to say "this went well" reached
for a primary tint and told people their work had gone wrong.

A notice now says what it means and gets the colour that goes with it, which
is the only arrangement where that stops happening.

A patch already carries the line numbers on both sides, so they are read out
of it rather than counted. Anything said about a line is drawn under that
line, which is where a person expects to find it and where they can act on it
without holding a file path in their head.

A badge says what something is; a chip says what somebody picked. The
difference is that the second one comes off. A list of choices with no way
back out of them is a list of decisions, which is a worse thing to hand
anybody.

A suggestion that says what is wrong and not what to write instead leaves the
reader to do the work twice. Where whoever said it offered the replacement,
it is drawn under the comment, against the line it replaces.

Six of them. Where something stands and how far along it is, a list with
nothing in it yet and one waiting to be filled, a part of a longer
document, and which repository an item in a mixed list came from.

Nine hand rolled empty states across the local views said the same thing
at three different paddings, and a verdict was a badge tinted with the
review pillar's green whatever it said.

A header takes a line of its own under the title, for what is true of
the thing being looked at rather than what can be done to it. A chip
gains the warning tone the other three already had. A diff shows what a
suggestion replaces beside what it proposes, and renders what was said
about a line as the markdown a model writes.

This package ships source rather than a build, so nothing fails here when
a component is renamed or removed. It fails in whatever imports it, and
the export map is the only place that can be checked without one.