Step 5: Logging polish + strict-concurrency flag flip + DocC catalog update

[itsniper]

Goal

Final polish pass after the architectural refactor: tighten residual Sendable conformances in the logging layer, enable swift-version 6 + -strict-concurrency=complete everywhere, update the DocC catalog to match the new public surface.

Context

See the investigation report at docs/investigations/swift6-concurrency-audit-2026-05-13.md, sections Recommendations §C, Migration order Step 5, and Preventive Measures.

The Willow 7.0 upgrade (commit 3657f45) resolved the bulk of the logging-layer concurrency issues. Only small polish remains.

Deliverable (Step 5 of 5 in the migration plan)

1. ReliaBLEConfig explicit Sendable. Change public struct ReliaBLEConfig { … } to public struct ReliaBLEConfig: Sendable { … }. Compiler-synthesized today, but explicit is better as a public API surface.
2. Verify OSLogWriter synthesizes Sendable under -strict-concurrency=complete. If it doesn't (e.g., on a future SDK where OSLog's Sendable status changes), add @unchecked Sendable (matches upstream Willow.OSLogWriter):

   public final class OSLogWriter: LogModifierWriter, @unchecked Sendable { … }

3. Enable Swift 6 mode + complete strict concurrency in Package.swift for both ReliaBLE and ReliaBLEMock targets:

   .target(
       name: "ReliaBLE",
       swiftSettings: [
           .enableExperimentalFeature("StrictConcurrency"),
           .swiftLanguageMode(.v6),
       ]
   )

4. DocC catalog update. Add a new Concurrency.md page documenting:
   • ReliaBLEManager is Sendable and callable from any actor (MainActor SwiftUI and background actors).
   • All actions are async.
   • All event surfaces return fresh per-subscriber AsyncStreams.
   • SwiftUI consumers should use .task { for await … } patterns.
   • BluetoothActor is internal; consumers should not reference it directly.
5. Architecture diagram (optional) in DocC showing the isolation graph (ReliaBLEManager → BluetoothActor → delegate shim → CB).
6. Smoke test: construct a ReliaBLEManager inside Task.detached and exercise every public method. See Tests/ReliaBLETests.
7. Peripheral Sendable test: capture a Peripheral in a detached task; failure to compile catches accidental non-Sendable field regressions.

Decision: do NOT document the LoggingService.enabled race in DocC

The race is benign last-write-wins, inherited verbatim from Willow 7.0's deliberate upstream choice. A separate issue (itsniper/Willow#3) tracks the upstream fix; ReliaBLE will inherit it transparently when it lands. No ReliaBLE-side workaround or DocC note needed.

Acceptance criteria

• swift build -strict-concurrency=complete clean on both targets.
• swift test passes including the new smoke + Peripheral Sendable tests.
• DocC builds clean (swift package generate-documentation). New Concurrency.md page renders.
• ReliaBLEConfig: Sendable explicit.

Not in scope

• SwiftLint rules / CI wiring — that's NFR 11 (separate parent issue).

References

• Investigation report: docs/investigations/swift6-concurrency-audit-2026-05-13.md
• Upstream Willow tracker: Fix Logger.enabled data race (deferred from 7.0 Sendable migration) itsniper/Willow#3
• Parent: Update for Swift Concurrency in Swift 6 #10
• Depends on: Make BluetoothManager Swift 6 safe #13, Step 2 issue, Remove Combine #12, Step 4 issue