feat: Enhance @Settings macro and enable standalone @Setting usage

[couchdeveloper]

Overview

This PR significantly enhances the macro system to provide more flexibility and better code generation.

Key Changes

Enhanced @Settings Macro

The @Settings macro now generates the same infrastructure as AppSettingValues:

• Thread-safe Config struct with OSAllocatedUnfairLock
• Dynamic store and prefix properties with getters/setters
• Proper locking for concurrent access

Standalone @setting Support

@Setting can now be used without requiring @Settings on the container:

// Works without @Settings!
enum MyDefaults {
    @Setting static var count: Int = 0
}

Type-Safe Container Resolution
Introduced __ContainerResolver using Swift's conditional conformance:

Automatically detects if container conforms to __Settings_Container
Falls back to UserDefaults.standard with empty prefix if not
Type resolution happens at compile time (zero runtime overhead)
Improved Nested Container Support
Fixed qualified name generation for nested containers:

Correctly generates names like "Profile::setting"
Works for both direct nesting and extension-based nesting

Breaking Changes

None - all existing code continues to work unchanged.