fix: nightly macOS Sendable + WinUI PresentationMapper compile

[enricopiovesan]

Summary

• Nightly 2026-08-26 required jobs failed: native-macos (Swift 6 Sendable on Any? presentation snapshots) and native-windows (TraverseStarter.Tests compiled EmbeddedHost.cs without PresentationMapper.cs).
• Mark presentation mapper structs @unchecked Sendable on all four Swift cores (equality already ignores runtime JSON; UI still does not compute title/tags/status).
• Link PresentationMapper.cs into starter WinUI tests, matching doc-approval / meeting-notes / loop.

Governing Spec

• Specs 001/002 presentation mapping; public embedder only.
• No business fields in UI. No sidecar.

Project Item

• Ticket ID: fix-nightly-native-macos-windows
• Item: PVTI_lADOEbiBt84BbzAzzg4Iq4I
• Status: In Progress (Cursor)
• Evidence: https://github.com/traverse-framework/reference-apps/actions/runs/32940364814

Definition of Done

• Four Swift cores Sendable-safe for nightly xcodebuild
• TraverseStarter.Tests.csproj includes PresentationMapper.cs
• No sidecar / fake runtime
• Nightly native-macos + native-windows green after merge (or re-run workflow_dispatch)

Validation

• cd apps/traverse-starter/TraverseCore && swift test — TraverseCore / PresentationMapper.swift compile; remaining test failure is pre-existing errorMessage setter (not this change)
• dotnet test not run here (NuGet restore failed locally)
• CI PR gates: repository-checks / quality / coverage-gate / embedded-smoke

Checklist

Traceability

• Project 2 claimed (Cursor, In Progress)
• AGENTS.md start-plan row
• Blocked MCP set unchanged

Production baseline (platform / runtime-client PRs)

• Embedded mode unchanged
• No new sidecar URL
• SDK doubles unchanged
• Docs: start-plan only
• Validation listed

Architecture

• No business logic added to UI layer
• No private Traverse internals imported