feat(storybook): still capture, story shots and visual diff

[H4ad]

What

Ported from SigNoz/signoz and adapted to this repo's Storybook.

• A Motion toolbar global with two values, Still (default) and Live, settleForCapture run as afterEach (so after a story's play), and the html.sb-still block in preview.css: zero-length single-iteration animations that keep their end state, no transitions, transparent caret.
• apps/docs/scripts/story-shots.mjs (pnpm -F docs shots) screenshots stories off a running dev server, one PNG per story and theme plus a shots.json recording what each shot is.
• apps/docs/scripts/story-shots-diff.mjs (pnpm -F docs diff:shots) pairs the PNGs of two runs by relative path and reports what moved, largest first, using Chromatic's comparison (YIQ distance over diffThreshold, antialiasing-only pixels ignored).
• .claude/skills/storybook-visual-diff drives both scripts.

Why

A snapshot of a story with a spinner or a caret catches it at whatever frame the capture landed on, so the same story reports a change on every run. Parking animations on their last frame is also where Chromatic parks them, so local shots and Chromatic agree. Live stays available for watching a transition in the browser.

The scripts give a local read on what a change moved before pushing to Chromatic.

How to verify

Run pnpm dev, open any story with motion (Spinner, Tooltip), and toggle the Motion control in the toolbar: Still holds the end frame, Live animates.

From apps/docs, shoot the same tree twice and diff it; the noise floor should be zero:

pnpm shots .story-shots/a --stories badge --theme dark,light
pnpm shots .story-shots/b --stories badge --theme dark,light
pnpm diff:shots .story-shots/a .story-shots/b .story-shots/diff

Needs ImageMagick (magick or convert) on PATH. Playwright is already a devDependency of apps/docs.