[P3] Plan: Nuke.* → Fallout.* namespace mapping

[ChrisonSimtian]

Goal

Document the exact 1:1 (or N:1) mapping table from current `Nuke.` namespaces to `Fallout.` BEFORE executing the rename. This is the planning step — implementation comes after.

Current namespaces

```
Nuke.Common
Nuke.Common.CI.*
Nuke.Common.ChangeLog
Nuke.Common.Execution
Nuke.Common.Git
Nuke.Common.IO
Nuke.Common.ProjectModel
Nuke.Common.Tooling
Nuke.Common.Tools.
Nuke.Common.Utilities
Nuke.Build
Nuke.Build.Shared
Nuke.Components
Nuke.GlobalTool
Nuke.MSBuildTasks
Nuke.ProjectModel
Nuke.SolutionModel
Nuke.SourceGenerators
Nuke.Tooling
Nuke.Tooling.Generator
Nuke.Utilities
Nuke.Utilities.IO.Compression
Nuke.Utilities.IO.Globbing
Nuke.Utilities.Net
Nuke.Utilities.Text.Json
Nuke.Utilities.Text.Yaml
```

Proposed mapping

Default: `Nuke` → `Fallout` prefix-swap. So `Nuke.Common` → `Fallout.Common`. Same for every other.

Question: opportunity to restructure?

Some natural consolidations worth considering:

• `Nuke.Build` + `Nuke.Build.Shared` + `Nuke.Common` overlap; do they merge?
• `Nuke.Utilities.IO.` could become `Fallout.IO.`
• `Nuke.Common.Tooling` + `Nuke.Tooling` overlap

Recommendation: keep the 1:1 swap for the rename PR. Restructuring is a separate concern — adds risk to an already-large change.

Done when

• Mapping table reviewed and approved
• Restructuring decisions captured (yes/no/later for each)
• Implementation issue opened

[ChrisonSimtian]

Mapping constraint update

Per the strategy chosen on #35, the bridge approach includes type-forwarding shim packages (Fallout.Compat.Nuke.*). This places a hard constraint on this issue:

The namespace mapping must be strict 1:1, no consolidation.

Reason: type-forwarding only works when the old type's full name exists in the new assembly. Consolidations (e.g. merging Nuke.Build + Nuke.Common → Fallout.Common) would orphan some Nuke.Build.X symbols and break the shim's ability to [TypeForwardedTo] them.

The "Question: opportunity to restructure?" section above is now answered: no. Restructuring is deferred to a post-rename, post-shim cleanup pass. This makes the rename mechanical and lets the migrator CLI (Fallout.Migrate) do a pure prefix-swap.

Locked mapping

Strict prefix swap. Nuke.X.Y.Z → Fallout.X.Y.Z for every namespace and assembly listed above. No exceptions.