ISO8601DateFormatter -> Date.ISO8601FormatStyle

[zunda-pixel]

Motivation

This package supports Codable date handling using ISO 8601 strings. Since the package targets cross-platform Swift, the implementation should fit well with Swift 6 strict concurrency and modern Foundation APIs.

ISO8601DateFormatter is a mutable reference type. It is commonly used as a cached shared formatter, but that pattern is awkward in Swift 6 concurrency because it can introduce Sendable issues or require unsafe workarounds.

Date.ISO8601FormatStyle is the newer value-style API for ISO 8601 formatting and parsing. Using it would make the implementation easier to reason about, more concurrency-friendly, and better aligned with modern Swift.

Proposed change

Replace internal uses of ISO8601DateFormatter with Date.ISO8601FormatStyle where possible, while preserving the current encoded date representation.

Important compatibility points:

• preserve UTC / Z behavior
• preserve fractional seconds behavior
• keep existing Codable round-trip behavior
• add tests for representative date encoding/decoding cases

Expected benefit

• Better Swift 6 strict concurrency compatibility
• Less reliance on shared mutable formatter state
• Fewer unsafe Sendable workarounds
• More modern Foundation API usage