test(11.1-06): plant one fixture per docs checker rule and run them in the gate

- testdata/clean passes every check; 63 violation cases each overlay one
  fault and a want.txt naming the single problem it must produce
- TestPlantedViolations also plants the forbidden name (split literals)
  and symlink escapes at run time; TestBuildOutputGuard covers --out
- check-phase11.1.sh --self-test runs the corpus through a go test -json
  detector that refuses FAIL, SKIP, zero tests and no tests to run
- the self-test scratch copy now carries examples/ and README.md, so its
  baseline sees the example command literals the real tree sees

internal/docsite/testdata/violations/site-duplicate-section/docs/site.yaml

@@ -0,0 +1,16 @@
+title: Demo
+description: "Demo is the clean docs checker fixture."
+base_url: ""
+edit_url: "https://forge.example/edit/{path}"
+source_url: "https://forge.example/src/{path}"
+llms_notes:
+  - "Every page of this fixture passes every check."
+sections:
+  - name: guide
+    title: Guide
+  - name: extras
+    title: Extras
+  - name: guide
+    title: Again
+  - name: api
+    title: API reference