fix(core): promote private Codable witnesses to fileprivate

[WendellXY]

Summary

@Codable (and @Decodable / @Encodable) on a private type generated private init(from:) / private func encode(to:), which fails to compile:

error: initializer 'init(from:)' must be as accessible as its enclosing type
       because it matches a requirement in protocol 'Decodable'
note: mark the initializer as 'fileprivate' to satisfy the requirement

A protocol-requirement witness must be at least as accessible as its enclosing type. A top-level private type is effectively fileprivate for conformance purposes, so a private witness can't satisfy Codable.

Changes

• Add a witnessSafe helper on DeclModifierSyntax that promotes private → fileprivate (preserving the original token's trivia) and leaves every other access level untouched. This mirrors what the Swift compiler does for its own synthesized Codable conformance.
• Apply it in both generation paths in CodableMacro.swift:
  • ExtensionMacro path (struct init(from:))
  • MemberMacro path (decodeModifiers / encodeModifiers for structs and classes)
• Since @Codable, @Decodable, and @Encodable all route through CodableMacro, the single fix covers all three.

Test plan

• Reproduced the original compile failure; confirmed gone after the fix.
• End-to-end decode/encode verified for private struct, fileprivate struct, private class, and private @Decodable / @Encodable.
• Added regression tests: private struct, fileprivate struct, private class.
• Full suite passes: 297 tests (was 294).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a Swift access-control issue in the @Codable / @Decodable / @Encodable macros where applying the macro to a private type could synthesize private protocol-requirement witnesses (init(from:) / encode(to:)) that fail to compile. The change introduces a helper to promote private witnesses to fileprivate (mirroring the compiler’s Codable synthesis behavior) and adds regression tests.

Changes:

• Added DeclModifierSyntax.witnessSafe to promote private → fileprivate for synthesized Codable witnesses.
• Applied witnessSafe in both macro generation paths (extension-based init generation and member-based encode/decode generation).
• Added regression macro-expansion tests covering private/fileprivate structs and private classes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Tests/CodableKitTests/CodableMacroTests+struct.swift	Adds regression tests asserting private struct witnesses are promoted to fileprivate and fileprivate remains unchanged.
Tests/CodableKitTests/CodableMacroTests+class.swift	Adds regression test asserting private class witness requirements use fileprivate access.
Sources/CodableKitMacros/SwiftSyntaxHelper.swift	Introduces DeclModifierSyntax.witnessSafe helper for access-level promotion while preserving trivia.
Sources/CodableKitMacros/CodableMacro.swift	Uses witnessSafe when generating synthesized init/encode members.

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