Render the design system as the values it is made of [#29]

[iderex]

Closes #29.

What was wrong

The design system was served from another repository as a specification, and this
build produced no page for it. The part that was missing was not the markup. The
file the values come from holds two different kinds of thing under one shape:
values a client has to meet, and sentences the file writes about itself so that
whoever opens it knows what they are looking at. A page rendering both does not
fit the document line the speed budget fixes, and a page deciding by key name
gets that file wrong, because it carries a weight that is a sentence and a
weight that is the number 540.

What this does

The page lists every leaf of the pinned copy that carries a value, decided by the
value and never by the key, and it prints the counts of what it listed and what
it did not so the split can be read off the page rather than out of the
generator. Where a value can be drawn it is drawn with that same value: a colour
is its own background and its own foreground at once, a radius is the corner it
makes, a type role is drawn at the size it is given, and each colour vision
preset's accent is drawn as the ring it makes in both schemes. Nothing on the
page states a size, a colour or a weight this repository chose.

Every value a drawing uses is read twice. This package reads it against one of
two anchored shapes first, a number that can stand in front of a unit or a
hexadecimal colour, and a value of any other shape is not drawn and is printed
with the path it sits at. Then the frame writes the property and hands the engine
the value, so it is read in a value position and filtered there as well. Nothing
converts a string into markup the engine writes out unread.

A group that can no longer be drawn reds the build and names itself. The file is
published elsewhere, so a renamed key is how that arrives, and a page that
silently stopped demonstrating what it says it demonstrates is worse than a red
build.

Both budgets are on the page under headings saying whose each one is. The client
numbers come from the same file the values do and the site numbers from the
constants the row that refuses a page reads, so neither table can go on stating a
limit nothing holds anybody to.

The rule, the two answers that were available and were not taken, and what each
of them would have cost are in
decisions/0014-what-the-design-system-page-renders.md.

The means is Go and the template package already in the toolchain. The page is
one more writer beside the four this build already has, it needs no dependency,
and the suites that exist judge it.

One thing the page does not draw

A type role is drawn at its size and states its weight in words. The property
that would draw a weight may not appear in what the build reads at all, which is
the row keeping a weight from being defined anywhere but the token file, and this
page is inside that row's subject like every other build input. So the
alternative was a page that weakens that row rather than a page that draws one
more thing, and the weight is stated beside the sample instead.

What the gate says

Run at a948fa4, the head this branch pushes.

go run . ci
gate: 7 legs, in order: format, vet, test, build, links, sitemap, invariants
  needs-network was not asked for. Asking costs a request to the public name from whatever machine runs it, and a verdict that moves when somebody else's service does rather than when this tree changes. Ask with: go run ./harness/needs-network
  format: ok, 52 file(s)
  vet: ok
  test: ok, 25 test file(s)
  build: ok, 8 file(s)
  links: every reference that stays inside this site resolves to a file the build wrote
  sitemap: every page the build wrote is listed once, and every entry has a page behind it
  invariants: ok, 36 rule(s) decided, 1 owed and not decided
7 of 7 legs ran. None was skipped.

go run . build | grep design-system
wrote dist/design-system.html (19173 bytes, 154 value(s) listed, 36 drawn, 52 sentence(s) not listed)

go run . reproduce | tail -2
reproduce: two builds of ., compared byte for byte
  8 file(s), identical in both builds

The guards, each shown going red for the reason it names

Five one-change near misses, each the mistake somebody actually makes, with the
tree restored between them.

The rule reading only the end of the value, which drops the half that catches a
leaf whose prose does not end where the leaf does:

go test ./internal/site -run 'Sentence|StatesHowMuch' -count=1
--- FAIL: TestASentenceThatDoesNotEndTheLeafIsStillProse (0.04s)
    designsystem_test.go:244: the page lists a leaf carrying prose, so the rule read only the end of the value
    designsystem_test.go:247: the page did not count the leaf carrying prose as one

A leaf holding nothing read as a value spelled null:

go test ./internal/site -run 'LeafHoldingNothing' -count=1
--- FAIL: TestALeafHoldingNothingIsCountedAsCarryingNoValue (0.04s)
    designsystem_test.go:227: the page does not count the leaf holding nothing, so it was read as a value or dropped in silence
    designsystem_test.go:230: the page lists the leaf holding nothing as though it were a value

The swatch handed a colour that is not the value beside it, which is the failure
that renders perfectly:

go test ./internal/site -run 'ColourIsDrawn' -count=1
--- FAIL: TestAColourIsDrawnWithTheColourItStates (0.04s)
    designsystem_test.go:305: the page does not draw the accent with itself, and a swatch built from a second copy is the failure this page exists against

A group that emptied rendering as a heading with nothing under it:

go test ./internal/site -run 'GroupThatCanNoLonger' -count=1
    --- FAIL: TestAGroupThatCanNoLongerBeDrawnRedsTheBuild/the_ring_width (0.04s)
        designsystem_test.go:347: the build accepted a token file the page cannot be drawn from
    --- FAIL: TestAGroupThatCanNoLongerBeDrawnRedsTheBuild/the_radius (0.01s)
        designsystem_test.go:347: the build accepted a token file the page cannot be drawn from
    --- FAIL: TestAGroupThatCanNoLongerBeDrawnRedsTheBuild/the_role_size (0.02s)
        designsystem_test.go:347: the build accepted a token file the page cannot be drawn from

A value of the wrong shape drawn anyway. The change disables the reading of a
number and leaves the reading of a colour in place, so two of the three cases go
red and the third is the one the other reading holds. That is the change being
one change rather than the case being weaker:

go test ./internal/site -run 'ShapeADrawing' -count=1
    --- FAIL: TestAValueThatIsNotTheShapeADrawingNeedsIsRefused/a_ring_width_carrying_more_than_a_number (0.01s)
        designsystem_test.go:376: the build accepted a token file the page cannot be drawn from
    --- FAIL: TestAValueThatIsNotTheShapeADrawingNeedsIsRefused/a_radius_carrying_a_unit_of_its_own (0.02s)
        designsystem_test.go:376: the build accepted a token file the page cannot be drawn from

Every fixture in that suite is a token file the suite wrote. A case asserting the
real copy's 154 values would be a statement about the copy on the day it ran, and
it would go red the next time somebody publishes a colour, which is the one event
this page has to survive without anybody editing it. The case that says so
directly adds a value and a sentence to a fixture copy and asks what the page
did.

A correction to a measurement this page was planned against

The reading that put the leanest rendering of the values at 7861 bytes measured
rows with no end tags. This repository reads every produced page strictly and
refuses an element that is not closed, so those rows are not available here and
the real list is larger. The list this page writes is 10890 bytes over the same
154 values, and the page fits anyway.

What is left over, and it is small

python -c "s=open('dist/design-system.html',encoding='utf-8').read();a=s.index('<dl>');b=s.index('</dl>')+5;d=s[a:b];print(20480-len(s), round(len(d)/d.count('<dt>'),1))"
1307 70.7

Run at a948fa4. So 1307 bytes are left under the document line and a value costs
about 71 of them, which is room for eighteen more values before the row that
refuses a page over the line refuses this one, by name. The record above states
that as the condition for reconsidering it, and the two answers it did not take
are what a refusal there would be a request to reconsider.

What this does not do

It does not reopen the address scheme and it does not move the document line.
Both were available answers to the same question and each moves a record that was
taken elsewhere, so neither is taken here.

It does not reproduce the motion the served page has. Those durations are not
values in the token file, so a page carrying them would be typing them into this
tree, and whether they become tokens is a question about that file rather than
about this page.

Reading

This change carries no second reader. The evidence above stands in place of one:
the gate at the pushed head, the build reporting what the page is made of, two
builds compared byte for byte, and each of the five guards shown going red on its
own one-change near miss.