docs(guide): Testing headlessly page

[blendsdk]

Part of #143 — the Guide expansion. Group Going further.

Purpose & audience

Teach testing an app without a terminal. The Introduction claims "assert on what is on screen in an
ordinary test run" and never shows how — this cashes that promise. Cheap page, high credibility.

The gap today

The headless-mount pattern is used all over the repo's own tests but is nowhere in user-facing docs.
A reader who wants to test their TUI in CI has no guidance.

Outline

1. Why it's possible: the engine is host-agnostic — no real TTY needed; you drive input and read the
   composed frame.
2. Mounting headlessly: build the app with a headless/viewport host and no native tty.
3. Reading the screen: app.loop.renderRoot.buffer().rows() gives the composed frame as strings —
   assert on what's actually painted.
4. Driving input: dispatching keys/commands (app.loop.dispatch(...)) and asserting the result.
5. Async + modal testing: awaiting an execView dialog headlessly, resolving it, asserting after.
6. createRenderRoot({ schedule }) for a bare view (no app), and the trap: that counter conflates
   repaint with relayout — assert a reflow via an inline literal on view.host, not the schedule
   counter.

Code examples to write

• Mount an app, dispatch a key, assert a row of buffer().rows().
• Drive a dialog: open via execView, dispatch Enter, await, assert the resolved value.
• A bare-view render-root test with createRenderRoot.

Live examples

• Reuse: none — a test-authoring page; a live xterm can't show a vitest run.
• Build new: none. (The runnable artifact here is a copy-pasteable *.test.ts, not a Play example.)

Source pointers

• packages/ui/src/view/ — createRenderRoot / RenderRoot (buffer().rows()).
• Memories ui-relayout-test-seam (no ViewHost double in ui tests; assert reflow on public
  view.host, never the createRenderRoot({schedule}) counter — it conflates repaint with relayout)
  and jsvision-ui-modal-dialog-patterns (the headless async/nested-confirm test harness).
• The docs-site's own test/paint-smoke.spec.test.ts mounts every example headlessly — a working
  reference for the mount pattern.

Constraints

• No .layout = (snippet-drift). Test files have their own conventions, but this page is a teaching
  artifact and is held to the guard.

Acceptance criteria

• Headless mount + buffer().rows() assertion shown; input driving shown.
• Async/modal headless testing covered; the render-root schedule-counter trap documented.
• Cross-links Debugging (shared frame-dump trick) and Writing your own widget.
• Sidebar under Going further; build clean.