Move ExecutionStatus and ITargetModel into the Fallout.Core.Execution namespace

[ChrisonSimtian]

Problem

ExecutionStatus and ITargetModel ship in Fallout.Core.dll but declare
namespace Fallout.Common.Execution (src/Fallout.Core/Execution/ExecutionStatus.cs:1,
src/Fallout.Core/Execution/ITargetModel.cs:3). Core is the innermost layer. A Core type
sitting under an outer layer's namespace inverts the layering in naming. Both types are
public, so correcting this breaks consumers and must ride a yearly major.

Outcome

Both types live under Fallout.Core.Execution. The architecture fitness test that guards
this rule has no exceptions left. Consumers upgrade by running fallout-migrate, without
hand-editing their own source.

Acceptance criteria

• ExecutionStatus and ITargetModel declare namespace Fallout.Core.Execution.
• GrandfatheredNamespaces in tests/Fallout.Core.Specs/ArchitectureFitnessSpecs.cs
  is empty, and the filter that reads it is removed.
• fallout-migrate rewrites the old namespace in consumer *.cs files. The rule goes
  into RewriteCsFilesStep, the step that already covers *.cs — not a new step. See
  the migration step recipe.
• tests/Fallout.Migrate.Specs covers that rewrite: the using form, the fully
  qualified form, and a file that must not be touched.
• CHANGELOG.md records the break under the next yearly major. It gives the old and
  new full type names and points at fallout-migrate.

Notes

The rule and the frozen exception list come from #586, which moved Fallout's constants into
Core. Breaking changes follow AGENTS.md rule 1. A shim cannot cover this rename: type
forwarders keep the full type name, and ExecutionStatus is an enum, so it cannot be
aliased. That makes the fallout-migrate rule the migration path, not an optional extra.