[Jeehut]

Summary

Adds an indent: overload for embedding error chains in indented text. Every line after the first receives the prefix, including multiline messages and Windows or Unicode line breaks.

ErrorKit.errorChainDescription(for: error, indent: "    ")
try await MyError.catch(isolation: #isolation) { /* actor-local operation */ }

Adds an explicit actor-isolated async catching overload. The original formatter and async catching signatures remain available. The contribution from #36 stays in the commit history, followed by separate adjustments.

The isolated overload accepts actor-local non-Sendable captures without adding @Sendable. A defaulted parameter on the original signatures was rejected because it breaks typed function references.

Draft status: Not approved for merge. The source-compatibility finding and compiler/platform gaps below remain open.

Source compatibility: let f = ErrorKit.errorChainDescription becomes ambiguous. Use ErrorKit.errorChainDescription(for:) or an explicit function type. This change is not fully source-compatible and needs a release-note migration warning.

Evidence

• Before the newline repair: 14 failures, covering seven non-LF separators at two nesting depths. After: 32 tests in 7 suites passed, including all eight separators.
• Tests ran with Apple Swift 6.4.2 on macOS arm64, using native SwiftPM and one build job. The final 32-test runs did not pass --parallel.
• Negative controls detected lost actor isolation, wrong or double wrapping, and missing deep indentation. The restored implementation passed.
• Typed legacy async and formatter function references passed regression tests. Empty indentation preserves the original formatter output.
• DocC conversion exited 0. The generated pages resolved both formatter overloads and all three catching overloads. Unrelated DocC warnings remain.
• Final external runtime recheck on 12ceaf82791448022c60bef20cf3cc6d53fb41ab: 149 checks passed, 0 failures, exit 0. The public-API client checked all eight separators, nested chains, empty lines, trailing separators, and multiline prefixes byte-for-byte. Scope: macOS arm64, Swift 6.4.2 Debug.
• The --disable-testable-imports suite failed on the pre-existing @testable import in ThrowableTests.swift:3. It is not counted as passed.
• Not checked: Swift 6.0, other platforms, Release optimization, or Thread Sanitizer. An existing mapper Sendable warning remains.

Merge Danger

Door: one-way (public API) · Blast Radius: ErrorKit clients.

Docs/Agents/BlastRadius.md is missing, so complete impact coverage is not established. Independent code reviews checked the API and the final newline repair. No release is part of this PR.