Hide Avatars Beyond Visibility Distance

## Parent Issue
Part of #1085 (Rendering Optimizations)

## Summary
Implement distance-based visibility for avatars to avoid rendering players that are too far away to be meaningfully visible. This is a simple visibility toggle, not a LOD (Level of Detail) system.

## Background
Rendering distant avatars consumes significant GPU resources (skinned meshes, multiple materials, accessories) while providing little visual value. A straightforward solution is to hide avatars beyond a configurable distance threshold.

## Implementation Details

### Target File
- `godot/src/decentraland_components/avatar.gd`

### Approach
1. Calculate distance from local player to each avatar every frame (or throttled)
2. Toggle avatar visibility based on distance threshold
3. Use hysteresis to prevent rapid toggling at boundary

### Suggested Implementation
```gdscript
const AVATAR_MAX_DISTANCE := 50.0
const AVATAR_HYSTERESIS := 5.0 # Prevents flickering at boundary

var _is_hidden_by_distance := false

func _process(_delta: float):
var local_player_pos = get_local_player_position()
var distance = global_position.distance_to(local_player_pos)

if _is_hidden_by_distance:
# Show again when closer (with hysteresis)
if distance < AVATAR_MAX_DISTANCE - AVATAR_HYSTERESIS:
_is_hidden_by_distance = false
visible = true
else:
# Hide when too far
if distance > AVATAR_MAX_DISTANCE:
_is_hidden_by_distance = true
visible = false
```

### Considerations
- The local player's own avatar should never be hidden
- Consider using Godot's visibility range instead for GPU-level culling
- Distance check can be throttled (every 0.5s) if performance is a concern

## Expected Impact
- Reduced draw calls in crowded scenes
- Lower GPU load from skinned mesh rendering
- Lower memory bandwidth from avatar textures

## Acceptance Criteria
- [ ] Avatars beyond 50m from local player are hidden
- [ ] Hysteresis prevents flickering at distance boundary
- [ ] Local player avatar is never hidden
- [ ] Distance threshold is configurable (optional)