fix(web-ui): prevent terminal output flicker on expansion

[wsp1911]

Summary

• Prevent terminal output from oscillating between adjacent pixel heights after expanding an ExecCommand card.
• Keep the normalized fallback output visible until xterm has rendered its first content frame.
• Share terminal text normalization, font metrics, and height calculation between the lazy fallback and xterm renderer.
• Add regression coverage for normalized fallback rows.

Type and Areas

Type:

Regression fix / UI/UX

Areas:

Web UI, Flow Chat, terminal output rendering

Motivation / Impact

• Expanded ExecCommand cards could flicker indefinitely because xterm's rounded cell height was written back into the React container from its ResizeObserver.
• After fixing that loop, a one-time flash remained while the lazy fallback was replaced by an initially empty and differently sized xterm renderer.
• Terminal output now remains visually stable throughout expansion.
• The xterm runtime remains lazily loaded, preserving the existing startup performance boundary.

Verification

• pnpm run type-check:web
  • Passed.
• pnpm --dir src/web-ui run test:run src/tools/terminal/components/LazyTerminalOutputRenderer.test.tsx src/flow_chat/tool-cards/ExecProcessToolCardView.test.tsx src/app/startup/startupPerformanceContract.test.ts
  • 3 test files passed.
  • 54 tests passed.
• git diff --check
  • Passed.
• Manually expanded the affected ExecCommand card.
  • Confirmed that continuous and one-time output flicker no longer occurs.

Reviewer Notes

• The shared presentation helper mirrors xterm's font-metric and device-pixel-ratio calculations without importing the xterm runtime.
• ResizeObserver still refits xterm when the container changes, but no longer feeds rounded cell heights back into React state.
• The plain-text fallback remains mounted until xterm reports a rendered content frame.
• No user-facing strings, migrations, or compatibility changes are included.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. No copy changes were required.