Generate CodingKeys enum in @Entity macro

[colemancda]

Summary

• @Entity now generates a public enum CodingKeys: String, CodingKey, Sendable, CaseIterable with a case for id plus every @Attribute/@CompositeAttribute/@Relationship property, in declaration order.
• Backwards compatible: if the type already declares a CodingKeys enum (or typealias), the macro skips generation and the manual declaration is used.
• The String raw type keeps the generated enum compatible with Embedded Swift, where CodingKey synthesis without a raw type is unavailable.

Limitation

When entity A's @Relationship(inverse: .foo) references entity B's keys, the compiler must type-check .foo against B.CodingKeys while macros are still expanding, and it will not expand @Entity's generated enum to satisfy another macro's argument. Entities referenced as a relationship destination therefore still need a manual CodingKeys; this is documented in the test model.

Testing

• New macro tests: generated enum shape and case order, and that a manual CodingKeys suppresses generation.
• Facility in the test model now relies on the generated enum, confirming Codable synthesis picks it up.
• Full suite passes: 255 tests in 27 suites.

[github-code-quality]

Code Coverage Overview

Languages: Swift

Swift / code-coverage/llvm-cov

The overall line coverage in commit c6f4b47 in the feature/coding-keys-... branch remains at 96%, unchanged from commit e43de80 in the master branch.

Show a line coverage summary of the most impacted files.

File	master e43de80	feature/coding-keys-... c6f4b47	+/-
Sources/CoreMod...os/Entity.swift	100%	99%	-1%