Implement the virtualizer package for viewport virtualization over large terminal text output. This covers PRs 1–3 of the implementation plan: - PR 1: Export createDisplayWidth from renderer (WASM per-codepoint wcwidth, R.WIDTH.* tests) - PR 2: Virtualizer core — appendLine, resolveViewport, ring buffer, ANSI scanner, wrap walker, getLineDisplayWidth (~66 tests) - PR 3: scrollBy + scrollToFraction with all deferred scroll-dependent tests (~92 tests total) resize() still throws "not implemented" — deferred to PR 4.