test: complete controlled view test foundation

[ddebowczyk]

Summary

• add a deterministic jsdom runtime with isolated storage, geometry, observers, clock, and UUIDs
• test controlled view interactions and multi-view portal and observer isolation
• replace the portal lookup with an instance-owned ref and validate non-empty view IDs
• declare supported Node versions and add a Node 20/22/24/26 CI matrix

Validation

• npm test
• npm run lint
• npm run build:lib
• npm run typecheck:consumer
• npm run typecheck:tauri-example
• npm run check:package
• npm pack --dry-run
• markdownlint README.md docs/**/*.md

This is a merge-ordered follow-up to #39. The focused change is commit 6c75666.