Add Automatic Directional Navigation Graph Generation (bevyengine#21668)

## Objective

Resolves bevyengine#21661 

Adds automatic directional navigation graph generation based on UI node
positions and sizes, eliminating the need for tedious manual graph
construction in dynamic UIs.

## Solution

Implements a spatial navigation algorithm that automatically computes
the nearest neighbor in each compass direction for UI elements, while
respecting any manually-defined edges.

### Features

- **Automatic edge generation**: Finds the best neighbor in each of 8
compass directions based on distance, alignment, and overlap
- **Manual override support**: Manual edges always take precedence over
auto-generated ones
- **Configurable**: `AutoNavigationConfig` resource allows tuning
alignment requirements, distance limits, and preference weighting
- **Opt-in**: Entities must have `AutoDirectionalNavigation` component
added to use, and therefore **not** a breaking change
- **Generic**: Core algorithm works with any `Vec2` position/size data,
not just `bevy_ui`

### Implementation

**New Components & Resources**
(`bevy_input_focus/src/directional_navigation.rs`):
- `AutoDirectionalNavigation` - Marker component to enable
auto-navigation
- `AutoNavigationConfig` - Configuration resource with settings:
- `min_alignment_factor`: Minimum perpendicular overlap (0.0-1.0)
required for cardinal directions
  - `max_search_distance`: Optional distance limit for connections
  - `prefer_aligned`: Whether to strongly prefer well-aligned nodes

**Core Algorithm**:
```rust
pub fn auto_generate_navigation_edges(
    nav_map: &mut DirectionalNavigationMap,
    nodes: &[(Entity, Vec2, Vec2)],  // (entity, center_pos, size)
    config: &AutoNavigationConfig,
)
```

For each node and each direction:
1. Filter candidates that are actually in that direction (cone-based
check)
2. Calculate overlap factor for cardinal directions (horizontal overlap
for N/S, vertical for E/W)
3. Score candidates based on:
   - Distance (closer is better)
   - Alignment with direction vector (more aligned is better)
   - Overlap factor (must meet minimum threshold)
4. Select the best-scoring candidate as the neighbor

**Scoring Formula**:
```
score = distance + alignment_penalty
where alignment_penalty = (1.0 - alignment) * distance * 2.0
```

This makes misaligned nodes significantly less attractive while still
considering distance.

### Usage

**Before (manual)**:
```rust
// Must manually specify all connections
for row in 0..N_ROWS {
    let entities_in_row: Vec<Entity> = (0..N_COLS)
        .map(|col| button_entities.get(&(row, col)).unwrap())
        .copied()
        .collect();
    directional_nav_map.add_looping_edges(&entities_in_row, CompassOctant::East);
}

// Repeat for columns...
for col in 0..N_COLS {
    let entities_in_column: Vec<Entity> = (0..N_ROWS)
        .map(|row| button_entities.get(&(row, col)).unwrap())
        .copied()
        .collect();
    directional_nav_map.add_edges(&entities_in_column, CompassOctant::South);
}
```

**After**:
```rust
// Just add the `AutoDirectionalNavigation` component!
commands.spawn((
    Button,
    Node { /* ... */ },
    AutoDirectionalNavigation::default(),
    // ... other components
));
```

## Testing
- Added new example: `auto_directional_navigation`
- Ran existing `directional_navigation`

## Showcase

### New Example: `auto_directional_navigation`

Demonstrates automatic navigation with irregularly-positioned buttons.
Unlike a regular grid, these buttons are scattered, but auto-navigation
figures out the correct connections - also shows currently focused
button, and the last "input" pressed to show the logical flow of
navigating:

```bash
cargo run --example auto_directional_navigation
```



https://github.com/user-attachments/assets/8dce1d8d-53b5-41b8-bc9a-7e32067f7978




**Key differences from manual `directional_navigation` example**:
- No manual `add_edges()` or `add_looping_edges()` calls
- Buttons positioned irregularly (not in a perfect grid)
- Works with absolute positioning and dynamic layouts

## Migration Guide

No breaking changes - this is a purely additive feature.

To adopt automatic navigation:
1. Add `AutoDirectionalNavigation` component to focusable entities
2. Optionally configure `AutoNavigationConfig` resource

Author: jbuehler23

Cargo.toml

@@ -4704,6 +4704,17 @@ description = "Demonstration of Directional Navigation between UI elements"
 category = "UI (User Interface)"
 wasm = true
 
+[[example]]
+name = "auto_directional_navigation"
+path = "examples/ui/auto_directional_navigation.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.auto_directional_navigation]
+name = "Automatic Directional Navigation"
+description = "Demonstration of automatic directional navigation graph generation based on UI element positions"
+category = "UI (User Interface)"
+wasm = true
+
 [[example]]
 name = "clustered_decals"
 path = "examples/3d/clustered_decals.rs"

crates/bevy_input_focus/Cargo.toml

@@ -61,10 +61,12 @@ libm = ["bevy_math/libm", "bevy_window/libm"]
 [dependencies]
 # bevy
 bevy_app = { path = "../bevy_app", version = "0.18.0-dev", default-features = false }
+bevy_camera = { path = "../bevy_camera", version = "0.18.0-dev", default-features = false }
 bevy_ecs = { path = "../bevy_ecs", version = "0.18.0-dev", default-features = false }
 bevy_input = { path = "../bevy_input", version = "0.18.0-dev", default-features = false }
 bevy_math = { path = "../bevy_math", version = "0.18.0-dev", default-features = false }
 bevy_picking = { path = "../bevy_picking", version = "0.18.0-dev", default-features = false, optional = true }
+bevy_ui = { path = "../bevy_ui", version = "0.18.0-dev", default-features = false }
 bevy_window = { path = "../bevy_window", version = "0.18.0-dev", default-features = false }
 bevy_reflect = { path = "../bevy_reflect", version = "0.18.0-dev", features = [
   "glam",