Update Nuke→Fallout codefix to map Common.ProjectModel → Solutions for v11

Why

The NukeMigrationCodeFix in Fallout.Migrate.Analyzers rewrites Nuke → Fallout token-by-token. Post-v11 (after the vs-solutionpersistence inlining in #248), this produces stale namespace paths:

Input (NUKE-era consumer code)	Codefix output today	Correct v11 target
using Nuke.Common.ProjectModel;	using Fallout.Common.ProjectModel; ❌ (namespace gone)	using Fallout.Solutions;
Nuke.Common.ProjectModel.Solution x;	Fallout.Common.ProjectModel.Solution x; ❌	Fallout.Solutions.Solution x;

Consumers who run fallout-migrate (or the codefix) on pre-rename NUKE code currently get code that doesn't compile against v11. The transition shim (Nuke.Components / Nuke.Common) does mirror Fallout.Solutions.* back to Nuke.Common.ProjectModel.*, so consumers can stay on the shim namespace longer — but the migration codefix should target the canonical v11 namespace.

Action

• Extend NukeToFalloutRewriter in src/Fallout.Migrate.Analyzers/NukeMigrationCodeFix.cs to handle the qualified-name case Nuke.Common.ProjectModel → Fallout.Solutions (visit QualifiedNameSyntax or post-process the rewritten tree)
• Restore the test expectation in tests/Fallout.Migrate.Analyzers.Tests/RealWorldSmokeTests.cs to Fallout.Solutions.Solution + using Fallout.Solutions; (currently reverted to Fallout.Common.ProjectModel.* to keep the test green during #248)
• Update TargetFalloutStub in that test file to also expose Fallout.Solutions.Solution (the inline stub only defines Fallout.Common.ProjectModel.Solution)
• Audit other rename-era namespace pairs the codefix might miss (anything else that moved out of Fallout.Common.* in v11)

Done when

• A NUKE consumer running the codefix gets v11-correct namespace paths
• RealWorldSmokeTests.RewritesRealisticPreRenameBuildClass expectations reflect the v11 target namespaces
• No regression on simple Nuke → Fallout cases with no ProjectModel/SolutionModel involvement