Visual testing 6/8: publish the report as a Storybook concern #716

Parent: #588 — step 6 of 8.

Problem

The visual report has a Storybook status page, and it is currently unmounted and unmountable.

It is mounted by three hand edits to a library's Storybook host config: a story glob, a static-directory entry, and a filesystem-allow addition. specs storybook init now scaffolds that file and --force rewrites it, so those three edits are edits waiting to be destroyed — and in practice the page has been unmounted ever since the host became generated. Reading results means opening a markdown file on disk.

Potential solution(s)

• Publish the report as a generated page through the existing concern mechanism: a visualtesting concern registered alongside the others, detecting on whether the library has a report on disk and building the page when it does.
• Separate sections for components and compositions, because their pass criteria differ — compositions are advisory, and one combined table hides that.
• The page reads the report JSON over HTTP, so a re-diff updates it on reload with no republish.
• The scaffolded host already reserves the mount point, serving a visual-testing baselines directory when one exists. That line is currently the only mention of visual testing anywhere in the CLI; this is what makes it mean something. It points at the previously reserved path and needs repointing at where the rest of this work writes.

Acceptance criteria

• A library with a report gets the page with no edits to its Storybook host directory at all.
• specs storybook init --force followed by a publish leaves the page working — the regression this step exists to prevent.
• A library with no report does not get the page, and publish says nothing about it.
• Re-running the diff and reloading shows new numbers without a republish.
• Component and composition totals appear separately, with the composition section stating that its results are advisory.

Case data

• Territory: cli
• Size: m

Notes

Depends on step 5. Replaces the hand-wired page that lived in the test repo.