Add deterministic presentation mode

[raiseCatError]

Summary

• Add an explicit NMSH_DETERMINISTIC=1 presentation seam for fixed completion display time and stable shimmer/activity animation phase.
• Keep Vespyr in its open frame by skipping its ambient blink timer in deterministic mode.
• Preserve real shell event timestamps, measured command durations, PTY/session behavior, timers, external programs, and persisted data.
• Document current scope and exclusions for visual/integration tests and later VHS/demo tooling.

Verification

• Focused presentation/welcome/activity tests: 25 passed
• Full test suite: 596 passed
• npm run build: passed
• npm run typecheck: passed
• git diff --check: passed

Human terminal validation

Try NMSH_DETERMINISTIC=1 npm run dev in a terminal, run a short command repeatedly, and confirm the displayed completion clock and live shimmer/spinner phase are stable while elapsed duration remains real. Confirm the Vespyr welcome stays open-eyed. Also run once without the variable and confirm normal timing/blinking. Automated tests do not replace this terminal check.

Parking note

This PR is isolated future/tooling work for #173. Keep it parked while v0.6 / #175 is active. Do not merge automatically.

Refs #173