Visual smoke tests & CI integration #776

Summary: Add visual-regression smoke checks and tests to validate thumbnail generation and scene load with placeholders.

Acceptance Criteria

• Headless test runs replay for canonical fixture and captures screenshots without errors.
• Test runs generate-thumbnail and asserts PNG exists and is 120x68.
• Scene preload test asserts texture keys load.
• New tests run in CI and pass.

Minimal Implementation:

• Add Vitest tests calling demo/replay and generate-thumbnail with small fixture.
• Ensure tests run quickly for CI.

Deliverables: tests, CI runbook entry.