docs: dedicated testing guide page

[johnleider]

Context

PR #247 introduces `V0Error` / `isV0Error` as the canonical way to assert v0-thrown errors in user tests. There's no docs page today that covers testing patterns for v0 consumers — testing knowledge is scattered across `.claude/rules/testing.md` (which is contributor-private) and individual test files.

Ask

Add a dedicated testing guide page to `apps/docs`, probably under a new "Guides" section or a "Recipes / Testing" subsection.

Suggested scope

• Setup — recommended deps (Vitest, happy-dom or jsdom), config snippets.
• Mocking Vue DI — `vi.mock('vue', () => ({ provide, inject }))` pattern + `hasInjectionContext` extension.
• Mocking plugins — `global.plugins: [createThemePlugin(), ...]` in @vue/test-utils.
• Fake timers — `vi.useFakeTimers()` + `vi.advanceTimersByTime` for `useTimer`, `useDelay`, `usePopover`, etc.
• SSR-only tests — splitting `*.ssr.test.ts` with hoisted `vi.mock('#v0/constants/globals', ...)`.
• Asserting throws + cause shape — `isV0Error(err, 'V0_CONTEXT_MISSING')` narrowing pattern, full list of codes from `V0ErrorDetails`.
• Asserting warnings — `vi.spyOn(console, 'warn')` capture-and-assert pattern (PHILOSOPHY §9.2 says never silently swallow).
• Locale-safe assertions — `toBeDefined()` not exact text (PHILOSOPHY §5.5).

Reference material

• `packages/0/.claude/rules/testing.md` — current contributor-facing rules; a lot of this content can be lifted to a user-facing page.
• Example tests across `packages/0/src/composables/**/*.test.ts`.

Notes

• Originated as side feedback during PR refactor: attach structured cause to v0 throws #247 review — the dedicated page is meant to be the one place users go when they want to test v0-based code, including the new `isV0Error` pattern.