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/command-in-readme/modules/demo/README.md

@@ -0,0 +1,19 @@
+# demo
+
+Demo greets people by name.
+
+```go
+import "example.com/docfixture/modules/demo"
+```
+
+## Usage
+
+Call `demo.Hello` or build a `demo.Greeter` and call `demo.Greeter.Greet`.
+
+See the [start guide](../../docs/guide/start.md#install).
+
+## Testing
+
+```sh
+./bin/demo demo:nope
+```