Add Next root theme provider

[hta218]

Refs Weaverse/builder#2659.

Adds the root-owned theme settings store needed for Next global modules to mirror Hydrogen's withWeaverse() behavior.

• Add WeaverseNextRootProvider for app/layout.tsx, seeded by root-loaded published-mode theme settings.
• Make route-level WeaverseNextProvider adopt the ambient root store instead of creating a second store; fallback behavior is unchanged when no root provider is mounted.
• Keep page runtime/renderer route-scoped while sharing the same theme store with root modules and Studio updates.
• Document the root provider pattern and add package tests.

Verification:

• pnpm --filter @weaverse/next test -- __tests__/next-adapter.test.tsx
• pnpm --filter @weaverse/next typecheck
• pnpm --filter @weaverse/next build
• pnpm exec biome check packages/next/src packages/next/__tests__ packages/next/README.md --diagnostic-level=error
• git diff --check

Note: POC/starter wiring is intentionally next slice; this PR is SDK package-level coverage.