feat: NE0003 — only one namespace declaration per file

[samtrion]

Summary

Implements NE0003: a file should declare exactly one namespace. A file that declares more than one namespace (sibling or nested) hides types from the name↔location mapping the other organization rules establish, and complicates NE0001/NE0002.

Stacked on #12 (NE0002). It reuses the shared FolderNamespace helper introduced there. Review/merge #12 first; the base will retarget to main once #12 lands.

Non-compliant

// Types.cs — two sibling namespaces
namespace Geometry.Shapes { public sealed class Circle { } }
namespace Geometry.Solids { public sealed class Sphere { } }   // NE0003

// nested namespaces
namespace Geometry { namespace Shapes { public sealed class Circle { } } }  // NE0003 on the inner one

Behavior

• Flags every namespace declaration except the first (in document order), for both sibling and nested shapes.
• Honors the file-organization opt-outs (PublishSingleFile, NetEvolveAnalyzerDisableFileOrganizationRules) and skips generated code.

Code fix

• Nested namespaces are flattened into a single file-scoped namespace whose value matches the folder-derived namespace (via FolderNamespace), falling back to the concatenated nesting (e.g. Outer.Inner) when no RootNamespace/ProjectDir anchor is available.
• Multiple sibling namespaces are intentionally left to NE0001's move-type fix, so no action is offered for that shape.

Unit + integration tests and docs/rules/NE0003.md included.

Closes #5

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[codecov]

Codecov Report

❌ Patch coverage is 98.07692% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.84%. Comparing base (26f0ef5) to head (fe69e60).

Files with missing lines	Patch %	Lines
...inability/SingleNamespacePerFileCodeFixProvider.cs	96.72%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   99.08%   98.84%   -0.25%     
==========================================
  Files           7        9       +2     
  Lines         328      432     +104     
  Branches       62       84      +22     
==========================================
+ Hits          325      427     +102     
- Partials        3        5       +2     

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