feat(persistence)!: inline vs-solutionpersistence; rename SolutionModel → Solution; namespace cleanup

[ChrisonSimtian]

⚠️ Breaking change — package IDs and namespaces change. Migration paths preserved via the Nuke.Common.ProjectModel.* transition shim.

Closes #248. Codefix follow-up tracked in #253.

This replaces the previous #254 which was auto-closed when its base branch (feat/consumer-compat-tests-on-main) was deleted on the #256 merge. Same commit content, now correctly based on main (which already has #256's consumer sentinels).

Affected surface

Was	Now
Fallout.SolutionModel (package, project, assembly)	Fallout.Solution at src/Persistence/Fallout.Solution/
Fallout.VisualStudio.SolutionPersistence (package, project, assembly)	Fallout.Persistence.Solution at src/Persistence/Fallout.Persistence.Solution/
Fallout.Common.ProjectModel.* namespace	Fallout.Solutions.* (plural per BCL convention)
Microsoft.VisualStudio.SolutionPersistence.{Model,Serializer,Utilities,...} namespace	Fallout.Persistence.Solution.{Model,Serializer,Utilities,...}
vendor/vs-solutionpersistence/ submodule	gone — sources inlined

Breaking-change detection result

The consumer-compat sentinels added in #256 caught this rename exactly:

• Fallout.Consumer.Local — broke (used Fallout.Common.ProjectModel + referenced src/Fallout.SolutionModel/). Resolved in this PR by updating the csproj path to src/Persistence/Fallout.Solution/ and the using to Fallout.Solutions. That update IS the documented consumer migration recipe.
• Nuke.Consumer — unaffected. The new Fallout.Solutions.* → Nuke.Common.ProjectModel.* shim mapping in ShimMarker.cs transparently covers the rename for NUKE-era consumers.
• Fallout.Consumer.NuGet — unaffected (pinned to Fallout.* 11.0.8, predates the rename).

So the blast radius is precisely what the shim's contract promised: only consumers using direct local Fallout source (no shim, no published packages) have to touch their code. Shim users and pinned-NuGet users see nothing.

Summary

• Vendored fork eliminated. The fork-of-fork-of-Microsoft (vendor/ submodule pointing at ChrisonSimtian/vs-solutionpersistence) is gone. 92 source files inlined into a new Fallout.Persistence.Solution project under src/Persistence/, owned outright by Fallout. No more submodule + fork-rebase coordination.
• Onion layering. src/Persistence/ is now the layered home for persistence-ring code. Future persistence-related projects go here.
• Facade renamed and rehoused. src/Fallout.SolutionModel/ → src/Persistence/Fallout.Solution/. The rebrand-era namespace mismatch (Fallout.Common.ProjectModel) is fixed to match the assembly name — Fallout.Solutions (plural to avoid Fallout.Solution.Solution awkwardness).
• Transition shim preserved. src/Shims/Nuke.Common/ShimMarker.cs now declares two mappings: the existing Fallout.Common.* → Nuke.Common.* plus a new Fallout.Solutions.* → Nuke.Common.ProjectModel.*. NUKE-era consumer code using using Nuke.Common.ProjectModel; [Solution] readonly Solution Solution; keeps compiling.

What's NOT in this PR

• Visibility narrowing. Parser types stay public for now. The [InternalsVisibleTo] decorations on Fallout.Persistence.Solution.csproj are future-intent. Per-type narrowing needs cascading CS0050 analysis — separate PR.
• Codefix update. Fallout.Migrate.Analyzers's Nuke→Fallout rewriter still produces Fallout.Common.ProjectModel.* (which is now a dead namespace). Test expectations temporarily reverted to match today's behavior. Tracked in Update Nuke→Fallout codefix to map Common.ProjectModel → Solutions for v11 #253.
• Dead-code prune. Inlined the full 92-file surface; auditing which files are actually used by Fallout's call graph is a follow-up.
• Archiving the old fork repo. Once this merges, ChrisonSimtian/vs-solutionpersistence is unreferenced. Archive (or transfer to Fallout-build once that's live) — tracked in chore(ci): drop Qodana #11.

Test plan

• dotnet build fallout.slnx — green, 0 warnings, 0 errors
• dotnet test fallout.slnx --no-build — all suites green
• Consumer-compat matrix (test(consumers): add NUKE + Fallout consumer compatibility sentinels #256) validated: Fallout.Consumer.Local updated (migration recipe), Nuke.Consumer + Fallout.Consumer.NuGet unaffected
• CI green on PR