Refactor: Flatten source tree under src

Added

- Flattened the library layout so the project now lives directly under src/\n- Updated the solution file to reference src/ModularityKit.Mutator.csproj

Changed

- Moved all library source files, abstractions, and runtime code out of the extra ModularityKit.Mutator/ wrapper directory
- Added the missing internal runtime helpers under src/Runtime/Internal to match the flattened structure
- Preserved the public API surface while aligning the repository layout with the new src/ convention

Result

The repository now keeps production code directly under src/ with the solution and project references aligned to the flattened layout, while the example and benchmark trees remain separate.

Author: rian-be

ModularityKit.Mutator.slnx

@@ -1,9 +1,15 @@
 <Solution>
-  <Folder Name="/Polygon/">
-    <Project Path="Polygon/BillingQuotas/BillingQuotas.csproj" />
-    <Project Path="Polygon/FeatureFlags/FeatureFlags.csproj" />
-    <Project Path="Polygon/IamRoles/IamRoles.csproj" />
-    <Project Path="Polygon/WorkflowApprovals/WorkflowApprovals.csproj" />
+  <Folder Name="/src/">
+    <Project Path="src/ModularityKit.Mutator.csproj" />
+  </Folder>
+  <Folder Name="/Examples/">
+    <Project Path="Examples/BillingQuotas/BillingQuotas.csproj" />
+    <Project Path="Examples/FeatureFlags/FeatureFlags.csproj" />
+    <Project Path="Examples/IamRoles/IamRoles.csproj" />
+    <Project Path="Examples/WorkflowApprovals/WorkflowApprovals.csproj" />
+  </Folder>
+  <Folder Name="/Tests/" />
+  <Folder Name="/Benchmarks/">
+    <Project Path="Benchmarks/ModularityKit.Mutator.Benchmarks.csproj" />
   </Folder>
-  <Project Path="ModularityKit.Mutator/ModularityKit.Mutator.csproj" />
 </Solution>