Test Next multi-runtime selection parity

[hta218]

Summary

• Add package-level tests for Next multi-runtime / nested instance parity.
• Lock the window.__weaverses registry/reuse contract against Hydrogen behavior: co-located page IDs, same-page/same-URL reuse, same-page/different-URL refresh, and Studio init per runtime candidate.
• Document that Builder owns active/leaf runtime selection; this SDK slice only ensures every candidate is registered and passed through.
• Add the Claude handoff + work-log entry under the existing Next Studio bridge spec.

Verification

• pnpm --filter @weaverse/next test -- __tests__/next-adapter.test.tsx — 5 files, 88 tests passed
• 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
• autoreview-copilot --mode local — clean, no accepted/actionable findings

Notes

Claude implemented the tests/work-log from the handoff; Hermes reviewed the diff, tightened wording around leaf selection ownership, and ran verification.

Tracker: Weaverse/builder#2659