fix(engine): preserve zero root opacity

[tianma-if]

Summary

• preserve an initial composition-root opacity of zero in DrawElement capture
• fall back to opacity 1 only for non-finite computed values
• add a DOM regression test for canvas injection

Why

The legacy root-opacity correction records the composition root opacity before wrapping it. The previous truthy fallback converted a valid opacity: 0 to 1, producing an incorrect correction ratio on later frames.

Validation

• bunx oxfmt and bunx oxlint on changed files
• bun run --filter @hyperframes/engine test -- drawElementService.dom.test.ts
• bun run --filter @hyperframes/engine typecheck