feat: NE0002 — namespace must match folder structure

[samtrion]

Summary

Implements NE0002: the declared namespace should match the folder structure, anchored at the RootNamespace MSBuild property. When a type's namespace does not follow the folder it lives in, the physical and logical layout diverge and navigation breaks.

Anchored at RootNamespace, every folder segment between the project directory and the file must appear as the corresponding namespace segment. A diagnostic is reported when the declared namespace does not equal RootNamespace + the relative folder path (.-joined).

Non-compliant

// RootNamespace = Geometry, file = Shapes/Primitives/Circle.cs
namespace Geometry.Shapes;   // NE0002: expected 'Geometry.Shapes.Primitives'
public sealed class Circle { }

Behavior

• Evaluates each top-level namespace declaration. Nested namespaces are left to NE0003; global-namespace files are out of scope.
• Files directly in the project root map to RootNamespace exactly.
• No mapping is attempted (and nothing is reported) when RootNamespace/ProjectDir are unavailable, the file is outside the project directory, or a folder segment is not a valid identifier.
• Honors the file-organization opt-outs (PublishSingleFile, NetEvolveAnalyzerDisableFileOrganizationRules) and skips generated code.

Code fix

Rewrites the namespace declaration to the expected value (WellKnownFixAllProviders.BatchFixer supported).

Notes

• Introduces the shared FolderNamespace helper (folder → namespace mapping) and exposes RootNamespace/ProjectDir via CompilerVisibleProperty. Both are reused by NE0003 (analyzer: NE0003 — only one namespace declaration per file #5), which is stacked on this PR.
• Unit + integration tests and docs/rules/NE0002.md included.

Closes #4

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (4d860df) to head (6757e2d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   98.59%   99.08%   +0.49%     
==========================================
  Files           4        7       +3     
  Lines         213      328     +115     
  Branches       44       62      +18     
==========================================
+ Hits          210      325     +115     
  Partials        3        3              

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