Migrate to Bevy 16 from Bevy 15

[ffmulks]

Migration to 16.1 with Claude help went smoothly. A suggested update to Readme.md and versioning is included. Other dependencies were updated as well while we are at it.

• Update Bevy dependency to version 16.1 in Cargo.toml
• Add hashbrown 0.15 dependency for Entry type compatibility

Breaking API changes fixed:

• Replace trigger.entity() with trigger.target() across all observers
• Update Parent to ChildOf with parent.get() → parent.parent()
• Implement new TriggerTargets trait methods returning iterators instead of slices
• Add required Mutability associated type to all Component implementations
• Update component hook signatures (remove third parameter from closures)
• Fix QueryEntityError enum usage with proper archetype parameter
• Update Once command structure to use FnOnce with proper Send bounds
• Remove Component reflection from Event types in event.rs
• Fix ReferenceType trait bounds for mutable references

Examples and tests:

• Update thirst example to remove LogPlugin dependency
• Add println! statements for console output visibility
• Fix test imports (remove hierarchy module references)
• All tests passing (12 unit tests + 11 doc tests)