feat(26-01): add coverlet.runsettings with declarative boilerplate exclusions

- Configure XPlat Code Coverage collector: cobertura format, SkipAutoProps,
  ExcludeByAttribute (Obsolete/GeneratedCode/CompilerGenerated/ExcludeFromCodeCoverage),
  ExcludeByFile globs for obj/, generated .g.cs, and the Program.cs bootstrap entrypoint
- Measured branch-rate baseline under new settings: 0.6913 (line-rate 0.9082)
- Gitignore TestResults/ build-artifact output

Author: Aitomates

.gitignore

@@ -11,6 +11,9 @@ obj/
 .vs/
 *.suo
 
+# Coverage / test run output
+TestResults/
+
 # NuGet
 *.nupkg
 packages/

coverlet.runsettings

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!--
+  Declarative coverlet configuration for the "XPlat Code Coverage" DataCollector.
+  Centralizes boilerplate-exclusion decisions (26-CONTEXT.md: 100% LOGICAL branch
+  coverage; framework boilerplate ignored via standard attributes/pragmas) instead
+  of scattering them across ad-hoc CLI flags.
+-->
+<RunSettings>
+  <DataCollectionRunSettings>
+    <DataCollectors>
+      <DataCollector friendlyName="XPlat code coverage">
+        <Configuration>
+          <Format>cobertura</Format>
+          <SkipAutoProps>true</SkipAutoProps>
+          <ExcludeByAttribute>Obsolete,GeneratedCodeAttribute,CompilerGeneratedAttribute,ExcludeFromCodeCoverageAttribute</ExcludeByAttribute>
+          <ExcludeByFile>**/obj/**,**/*.g.cs,**/Program.cs</ExcludeByFile>
+        </Configuration>
+      </DataCollector>
+    </DataCollectors>
+  </DataCollectionRunSettings>
+</RunSettings>