Deterministic presentation mode for visual and integration tests #173

Goal

Provide an opt-in test/development mode (for example NMSH_DETERMINISTIC=1) that makes NMSh-owned presentation reproducible without changing normal user behavior.

Stabilize where applicable

• Vespyr ambient timing and other time-dependent content.
• Animation clocks and random delays.
• Current-time-dependent render output.
• Unstable ordering in fixtures or presentation.

Keep the seam explicit and test-focused; do not globally freeze shell command behavior or mutate user settings. This mode should support the animation system and the later VHS/demo tooling issue.