feat(ui): add layout primitives and SSR smoke

[kattsushi]

Part of tracker #2. Targets feat/ui-package.

PR Type

• Bug fix
• New feature
• Documentation only
• Code refactoring
• Maintenance/tooling
• Breaking change

Summary

• Adds custom SSR-safe layout primitives for @os-copilot/ui.
• Adds mobile-first Storybook stories for the new primitives.
• Adds SSR/SPA smoke coverage and extends public import/export smoke coverage.

Chain Context

master
└── 📍 tracker #2: feat/ui-package
    ├── ✅ PR #3: scaffold package + shadcn routing
    ├── ✅ PR #4: Zaidan baseline + Storybook/CSS
    └── 🔎 PR #5: layout primitives + SSR/SPA verification

Scope

Included:

• Center
• Stack
• Grid
• Float
• GridPattern
• SSR/SPA smoke fixture/tests
• import/export smoke coverage for root + source-routed imports

Deferred intentionally:

• data-table / data-table-filters because they need a TanStack Table dependency/API decision.
• toast because it needs a Sonner dependency/API decision.
• copy-id-button because clipboard behavior and async feedback deserve separate review.
• marquee because motion/reduced-motion behavior deserves separate review.
• automated visual regression tooling because no Playwright/Chromatic dependency decision exists yet.

Changes

File	Change
packages/ui/src/components/ui/{center,stack,grid,float,grid-pattern}.tsx	Adds custom layout/decorative primitives.
packages/ui/src/stories/ui/*	Adds Storybook examples for each new primitive.
packages/ui/src/index.ts	Exports new primitives from the root package API.
packages/ui/src/testing/tree-shaking-smoke.test.ts	Extends public import smoke coverage.
packages/ui/src/testing/ssr-*.tsx	Adds SSR and SPA smoke coverage.

Test Plan

• pnpm exec dprint check packages/ui/src packages/ui/vitest.config.ts
• pnpm exec oxlint packages/ui/src
• pnpm nx test ui --run
• pnpm nx typecheck ui
• pnpm nx build ui
• pnpm nx build-storybook ui
• Fresh reviewer audit completed with no blockers.

Notes

• @os-copilot/ui/components/ui/* wildcard exports cover the new primitives. This PR does not add ergonomic subpaths like @os-copilot/ui/center; only button and chart currently have explicit ergonomic subpaths.
• Kobalte overlay SSR string rendering can hit getNextContextId cannot be used under non-hydrating context, so true SSR smoke coverage focuses on layout/decorative/Button while overlays remain covered by SPA smoke.
• Vitest/jsdom logs Window.scrollTo() method as not implemented during tests; tests pass.

Contributor Checklist

• Linked tracker PR feat(ui): track UI package chain #2
• Added exactly one type:* label
• Ran shellcheck on modified scripts (not applicable: no shell scripts changed)
• Skills tested in at least one agent
• Docs updated if behavior changed
• Conventional commit format
• No Co-Authored-By trailers