Step NC04: Conversation And Message Feature

[deku2026]

Summary

• Implements Step NC04: Conversation And Message Feature
• Keeps scope limited to the active step
• Updates traceability and acceptance evidence required by the step

Verification

• $env:MSBUILDDISABLENODEREUSE = "1"; dotnet test apps\desktop\ArcChat.Desktop.UiTests\ArcChat.Desktop.UiTests.csproj -m:1 --no-restore: passed (45 tests)
• $env:MSBUILDDISABLENODEREUSE = "1"; dotnet test packages\local-persistence\ArcChat.LocalPersistence.Tests\ArcChat.LocalPersistence.Tests.csproj -m:1 --no-restore: passed (11 tests)
• $env:MSBUILDDISABLENODEREUSE = "1"; dotnet test tests\ArcChat.Architecture.Tests\ArcChat.Architecture.Tests.csproj -m:1 --no-restore: passed (15 tests)
• $env:MSBUILDDISABLENODEREUSE = "1"; dotnet build ArcChat.slnx -m:1 --no-restore: passed (0 warnings, 0 errors)
• git diff --check: passed

Traceability

• Rows updated: NC-CORE-001, NC-FEAT-001, NC-FEAT-002, NC-FEAT-003, NC-FEAT-004, NC-FEAT-005, NC-FEAT-015, NC-FEAT-018, NC-FEAT-019, NC-FEAT-026, NC-FEAT-028
• Matrices updated: exporter goldens and protocol public API evidence updated in-repo; C# traceability rows reflected in this PR body
• Final-gate impact: enables FG.04 conversation proof and keeps FG.08 export/share evidence current

Out Of Scope

• Real provider onboarding beyond the existing model-provider foundation
• Full NC07 mask catalog/prompt management beyond NC04 new-chat recommendations
• MCP tools, artifacts, realtime audio, packaging, and settings-access work owned by later steps

Open Questions

• None