perf(ink): cache text measurements across yoga flex re-passes

[ethernet8023]

Why did i write this PR?

Scrolling in the new TUI the thinking info expanded was mad laggy

What does this PR do?

Adds a per-ink-text measurement cache keyed by width|widthMode to avoid re-squashing and re-wrapping the same text when yoga calls measureFunc multiple times per frame with different widths during flex layout re-pass

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• adds caching in ui-tui/packages/hermes-ink/src/ink/dom.ts

How to Test

1. Expand the thinking box while the model is doing a long thinking process
2. scroll up and down a lot

previously, you'd get a ton of lag (in yoga layout)
now, you get no lag :D

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: NixOS + fish + ghostty