Clarify progress rendering and Spectre capture guidance

[carldebilly]

Summary

This PR clarifies how Repl progress should be reported, rendered, and routed across console, hosted sessions, Spectre, and MCP.

It adds a centralized docs/progress.md page covering:

• IProgress<double>
• IProgress<ReplProgressEvent>
• IReplInteractionChannel progress helpers
• progress states and auto-clear behavior
• advanced terminal progress / OSC 9;4
• text fallback labels and TUI/Spectre live-surface guidance
• Repl progress vs Spectre.Console.Progress
• progress-related configuration
• MCP progress mapping

It also updates the Spectre capture examples to use session-aware IReplIoContext.Error instead of Console.Error.

Why

Progress behavior was documented in several places, but the full model was not easy to discover. This makes the intended pipeline explicit and calls out an important TUI caveat: Repl progress still has a text fallback label, so app-owned full-screen surfaces should capture or redirect regular Repl interaction feedback.

Validation

• Documentation-only change.
• Previously validated Spectre docs branch with:
  dotnet test --project src\Repl.SpectreTests\Repl.SpectreTests.csproj --no-restore

---

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 5 additional findings.

[carldebilly]

Addressed the review feedback in 5f44d27:

• Removed the hardcoded spectre help branch from CoreReplApp.Execution.
• Added an internal help-output factory registration point on OutputOptions.
• Registered the Spectre help render model from Repl.Spectre via UseSpectreConsole().
• Replaced the empty console-width catch in SpectreHumanOutputTransformer with Trace.TraceInformation(...) while preserving the fallback behavior.

Validation:

• dotnet test --project src\Repl.IntegrationTests\Repl.IntegrationTests.csproj --no-restore --filter "FullyQualifiedName~Given_HelpDiscovery|FullyQualifiedName~Given_OutputFormatting" -> 50 passed
• dotnet test --project src\Repl.SpectreTests\Repl.SpectreTests.csproj --no-restore -> 8 passed