Animals should sleep at night

### Is your feature request related to a problem? Please describe.

Animals in the game are always active regardless of time of day.
This breaks immersion - in reality, most animals have sleep cycles
and are either nocturnal or diurnal. A deer sleeping in the forest
at night, or a bear resting in its den, would make the world feel
much more alive and realistic.

### Solution you would like.

Animals should have a sleep/wake cycle based on time of day:
- Diurnal animals (deer, rabbits, birds) sleep at night,
less active and easier to sneak past
- Nocturnal animals (some predators) more active at night,
resting during the day
- Sleeping animals could be harder to detect the player
approaching, but wake up if noise threshold is exceeded

### Describe alternatives you have considered.

A simpler implementation could just reduce animal movement speed
and aggro range at night/day depending on type, without full
sleep state - as a first pass before proper sleep behavior.

### Additional context

This would also add gameplay depth - hunting sleeping animals
at night vs avoiding active predators during the day.
Would interact naturally with existing stealth and noise systems.

The issue names no files, tests, or entry points; start by locating the animal activity, time-of-day, stealth, and noise systems. Clarify the animal sleep cycles, detection behavior, wake-up threshold, and whether the simpler movement and aggro-range approach is the intended first step. Done means the agreed behavior is implemented consistently for relevant animals.

Written by the indexing model from the issue text.