docs: address #3236 review feedback (Cosmos default, stylesheet setup, CHANGELOG/ROADMAP corrections)

[showerbee]

Follow-up to #3236

Additional description

Docs-only follow-up to #3236, which merged before its review feedback was addressed. It covers @interactivellama's review comment and the findings from a second review pass. Each finding was checked against git history and the published tarballs.

README (docs(readme))

• Makes Cosmos the default theme, per @interactivellama's comment. The Highlights section and the stylesheet table now list Cosmos first.
• Fixes the Styling example. The old <link href="/node_modules/..."> only worked in Storybook, which maps dist/css to /slds2 in .storybook/main.ts. Installing the package doesn't serve node_modules over HTTP, so anyone copying that example got unstyled components. The section now shows a bundler CSS import first, then a serve-and-link alternative for setups without a bundler.
• Adds a note that the SLDS 2 stylesheet references images through ../../public/ URLs, and that @salesforce-ux/design-system-2 2.264.x doesn't ship them. Vite warns and builds anyway. webpack's css-loader fails unless its url option skips those paths. This gap is upstream, in the SLDS 2 package, and is worth raising with the SLDS team.

CHANGELOG (docs(changelog))

• alpha.3: alpha.2 already published with --tag alpha in the workflow. Chore/update package to use alpha tag #3220 made alpha the package default through publishConfig.tag.
• alpha.2: says components were migrated to TypeScript and many rewritten as function components. Some class components remain, for example AlertContainer and ToastContainer.
• alpha.2 and alpha.3 Removed: lists the irregular SLDSMenuPicklist alias and the SLDS-prefixed alias of each removed component. These were checked against the root exports of the 0.10.65 and alpha tarballs.

ROADMAP (docs(roadmap))

• forms has a nested story and test (forms/input/__docs__/inline, forms/input/__tests__/inline.test.jsx), so it's removed from the no-story and no-test lists.
• Story snapshot coverage is qualified. The snapshot glob __docs__/*.stories.* skips nested stories, so InlineInput and SearchInput are listed as a P2 gap.
• The popper-shape adapter is attributed correctly: it's computeFloatingData in utilities/dialog/index.jsx, and dialog-helpers.js only consumes its output.
• Removes "non-coverable content" from the VisualPicker gaps, because the coverable prop already exists.
• "No Storybook deployed" is now "no public v11 documentation site". Chromatic does build Storybook on push.

Not in this PR: @interactivellama also asked for Cosmos to be the default theme in Storybook. That one is for @Niznikr, so this PR doesn't touch .storybook/.

Next: a fix: PR to loosen the @salesforce-ux/design-system peer dependency, which is pinned to 2.25.0-alpha.2. That PR will also simplify the README's ERESOLVE troubleshooting.

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CONTRIBUTOR checklist (do not remove)

Please complete for every pull request.

Green gate — all of these must pass locally (CI runs the same):

• npm run lint passes with 0 errors (npm run lint:fix to auto-fix). Warnings are the tracked gradual-migration backlog; do not add new errors.
• npm run typecheck is clean (tsc --noEmit).
• npm test -- --run passes — this runs both Vitest projects (jsdom unit + real-browser browser) and the story HTML snapshots. If you changed rendered markup on purpose, update snapshots with npm test -- --run -u and review the diff.
• npm run build succeeds (type-check + Vite library build + .d.ts + entry shims).

Behavior & docs:

• First-time contributors: sign the CLA (a bot will comment with instructions).
• Added/updated Vitest + React Testing Library tests for the change (template: components/button/__tests__). New props and bug fixes should get a regression test. Tests that genuinely need a real browser (focus traversal, layout/measurement, canvas) go in a *.browser.test.jsx file (the browser project); everything else is jsdom. N/A: docs only.
• Reviewed the component's Storybook stories: npm run storybook → http://localhost:6007/. Confirm the story actually renders and behaves (deprecated Storyshots/Karma are gone; there is no separate browser-tests server). N/A: docs only.
• Story/example code uses the component's real prop API (e.g. render-props like onRenderContent / onRenderVisualPicker, icon props, events), not assumptions — a common source of "component looks broken" reports has been stories authored against the wrong API. Cross-check against components/<name>/__examples__. N/A: docs only.
• Storybook Accessibility (a11y) panel shows no new violations for the affected stories. N/A: docs only.
• If you added or changed a type, remember runtime PropTypes were removed in the TS migration — types are the contract. Ship accurate .d.ts (they're generated by the build); JS consumers rely on them via editor type-acquisition. N/A: docs only.

REVIEWER checklist (do not remove)

• CI is green (GitHub Actions CI workflow: lint, typecheck, test, build). Chromatic visual review, if configured, has been checked.
• Change has adequate test coverage; bug fixes include a regression test.
• Reviewed the affected Storybook stories and their a11y panels.
• Rendered markup still conforms to SLDS — check the story HTML snapshot diff (components/__tests__/__snapshots__/story-snapshots.test.jsx.snap).
• Public API / package surface: if exports changed, the deep-import contract still holds (@salesforce/design-system-react/components/<name> and nested paths resolve for ESM, CJS, and types). Removing/renaming a public component or export is a breaking change → feat!:/BREAKING CHANGE: and a major version bump.