test: automation-driven GUI test suite

[sepehr-safari]

Scope

Grow the S0 automation smoke (scripts/smoke.sh — launch, assert the tree
rendered, one screenshot) into a fuller automation-driven GUI test suite that
drives Studio's headline flows through its embedded automation server and
captures deterministic screenshots as CI artifacts.

Deliverables

• Driven flows (via native automate snapshot/assert/screenshot): open the
  sample trace → timeline renders → select an event → detail pane populates →
  failure panel; the filter bar; and the live-capture surface (switch to it,
  assert the controls render).
• A screenshot per major surface, uploaded as CI artifacts.
• Wired into the existing CI smoke job (Linux · Xvfb) — deterministic, no
  flakiness; assertions read the semantics snapshot (GPU-independent).

Acceptance criteria

• The suite drives the flows above and asserts on the rendered tree; screenshots
  land as non-empty artifacts.
• Green in CI on Linux (Xvfb) and runnable locally on macOS.

Dependencies

Builds on the S0 smoke (#6) and the S3–S5 UI. Independent of packaging.