refactor(storybook): rationalize sidebar taxonomy and prune variants

[danyelf]

PR checklist

• Ensure you have added or ran the appropriate tests for your PR.
• DCO signed

What type of PR is this?

refactor — storybook-only; no product code or user-facing behavior changes.

What this PR does / why we need it:

Storybook had grown to 16 top-level folders and ~428 stories, with two competing taxonomies (Visualizations/* vs top-level Schema/, Editor/, Lineage/), singleton folders for App/Editor/Schema/Summary/Timeline/Data, and a large variant explosion across primitives (EmptyState had 17, Toaster had 15, SquareIcon had 13 — most just demonstrating prop values already covered by argType controls). Finding features in the sidebar was hard.

This PR reorganizes the sidebar into 4 top-level buckets and prunes variant-explosion files down to a Default (with controls) plus the compositions that actually demonstrate something controls can't show.

Final sidebar shape:

Checks/      CheckList, CheckDetail, OutdatedCheckIndicator,
             TimelineEvent, ChangeSummary, EnvInfo
Diffs/
  Charts/    TopKBarChart
  Results/   Schema, RowCount, Profile, Query, ValueDiff,
             Histogram, TopK, SqlEditor, DataGrid
Lineage/
  LineageCanvas/   (LineageCanvas, CllExperience, ScaleTests/*)
  LineageLegend
  NodeView/        (NodeView, LineageTab)
Primitives/  (all formerly-split DiffPrimitives folded back in)

Notable prunes (16 → 3 pattern, with controls + compositions):

Component	Before	After
SquareIcon	13	3
DataTypeIcon	12	2 (kept AllCategories table)
ScreenshotBox	9	2
Split	12	4
DropdownValuesInput	16	3
ExternalLinkConfirmDialog	13	5 (3 are play tests)
MarkdownContent	18	2
EmptyState	17	5
Toaster	15	3
DiffText	16	3
RunStatusBadge	12	3
TimelineEvent	18	15 (constrained by visual.ts + .test.tsx)
EnvInfo	10	7

Other cleanup:

• Dropped tags: ["autodocs"] from every story file. The autodocs page duplicated the canvas and slowed Vite cold-start.
• Deleted HistogramDiffForm + TopKDiffForm stories — broken (require a LineageGraphContext the storybook mocks don't satisfy).
• Renamed HistogramResultView.stories.tsx → HistogramDiffResultView.stories.tsx (file now matches the exported component).
• Rewrote the docs-page smoke loop in verify-ui-stories.visual.ts (would have broken when autodocs was removed) to load story IDs directly.

Net: 428 → 244 stories.

Which issue(s) this PR fixes: N/A

Special notes for your reviewer:

• TimelineEvent kept 15 stories (not the originally-proposed 9) because 7 of them are pinned by TimelineEvent.visual.ts (Playwright visual regression) and 6 more are consumed by TimelineEvent.test.tsx via composeStories() as unit-test fixtures.
• The 12 errors that surface under pnpm --filter @datarecce/storybook exec tsc are MUI-augmentation drift, not regressions — they exist on main and are fixed by a possible follow-up tsconfig PR. Canonical cd js && pnpm type:check passes clean.

Does this PR introduce a user-facing change?:

NONE

[danyelf]

Responses to a self-review:

1. HistogramDiffForm + TopKDiffForm deletion (the only real signal). Both are production components, exported from @datarecce/ui/components and registered in run/registry.ts:29,42,188. The PR justifies deletion as "the storybook mocks don't satisfy LineageGraphContext." ... Deleting working coverage for two production form components on a "probably broken" hunch is the wrong direction.

The page did not render -- it showed an internal error that the LineageGraphContext mock wasn't working. I'd add ot that the forms themselves are barely load-bearing; they're not the pieces I'm most worried about inspecting.

2. verify-ui-stories.visual.ts docs-page loop removed. The pre-PR loop iterated 14 UI components and asserted no console/page errors on /docs/.... The replacement (5
   hand-picked smoke tests) loses passive error detection on 9 components. Commit 22b472c originally added this loop on purpose. If tags:["autodocs"] removal makes the
   docs URL meaningless, fine — but the loop could have been adapted to iframe URLs instead of deleted. Either restore it (rewritten for iframes) or note the deliberate
   coverage drop in the PR body.

Because we've had many broken stories in the test without actually turning this red, I'm not sure that it was doing what we wanted it to. I'm ok restoring it (and the autodocs), but I'd like to do so with intent.