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/clean/docs/index.md

@@ -0,0 +1,12 @@
+---
+title: Demo docs
+description: "The landing page of the clean fixture."
+section: index
+order: 0
+---
+# Demo docs
+
+Start with the [start guide](guide/start.md), then read [usage](extras/usage.md#call-it).
+
+> [!NOTE]
+> Every page of this fixture passes every check.