Use only FoundationEssentials when available

[t089]

• Import FoundationEssentials instead of Foundation when available (#if canImport)
• Gate all NS* types (NSNull, NSArray, NSDictionary, NSMutableArray, NSMutableSet, NSNumber, NSString) behind #if canImport(ObjectiveC) so the library compiles without the Objective-C runtime
• Replace NSNull with a new pure-Swift YAMLNull type for null value representation
• Replace NSRegularExpression with Swift Regex for timestamp parsing
• Replace DateFormatter with Date.ISO8601FormatStyle for date formatting
• Replace NumberFormatter-based float formatting with a generic formatYAMLFloat function using Swift's built-in LosslessStringConvertible
• Replace CharacterSet-based checks with Character property checks (e.g. isLetter, isNumber)
• Replace various Foundation methods (.replacingOccurrences(of:), .components(separatedBy:)) with Swift standard library equivalents (.replacing(), .split())