feat: source generator fail-safe (internal error diagnostics)

[Skymly]

Summary

• Add shared GeneratorFailSafe helper with ExecuteParse and TryEmit to convert unexpected generator exceptions into compile-time diagnostics instead of crashing Roslyn.
• Register per-domain internal error IDs: OBS2005, OBS3006, OBS4008, OBS5008, OBS6008, OBS7008, OBS8006, OBS9008.
• Wrap parse/emit pipelines across all eight domains; consolidate emit fail-safe in domain IncrementalValuesProviderExtensions.
• Fix Events OBS2005 message formatting for both parse and emit paths (MessageArg + MessageArg2).
• Add SignalR regression tests (unit tests + IInternalErrorProbe integration hook active only for GeneratorTests assembly).
• Mark ROADMAP P3-A E2 complete.

Closes #112

Companion docs PR: Skymly/Observables.Docs (internal error diagnostic tables).

Test plan

• dotnet build Observables.slnx -c Release
• dotnet test Observables.SignalR.R3.SourceGenerators.Tests --filter GeneratorFailSafeTests
• dotnet test Observables.Events.R3.SourceGenerators.Tests
• Nuke Test target (full manifest)

[Skymly]

Review comment

Solid E2 delivery. The shared GeneratorFailSafe split (ExecuteParse vs TryEmit) keeps domain generators thin, and consolidating emit fail-safe in each shproj IncrementalValuesProviderExtensions is the right place for it.

Highlights

• OperationCanceledException is rethrown on both paths — correct for incremental generators.
• Per-domain internal error IDs avoid colliding with existing *007 analyzer slots.
• Events MessageArg2 fix ensures OBS2005 reports type: message on both parse and emit paths.
• SignalR coverage is practical: unit tests for the helper plus an IInternalErrorProbe hook scoped to the GeneratorTests assembly.

Merge notes

• Companion docs PR: Observables.Docs#8 — merge after or together with this one.
• On the next release that ships these diagnostics, remember to move the new rules from each domain AnalyzerReleases.Unshipped.md into Shipped.md.

Verified locally (pre-PR)

• dotnet build Observables.slnx -c Release
• GeneratorFailSafeTests (3/3)
• Events R3 generator tests (16/16)
• Nuke Test target green