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/frontmatter-api-section/docs/api/extra.md

@@ -0,0 +1,9 @@
+---
+title: Extra
+description: "Extra page."
+section: api
+order: 90
+---
+# Extra
+
+Text.

internal/docsite/testdata/violations/frontmatter-api-section/want.txt

@@ -0,0 +1,3 @@
+rule: frontmatter
+file: docs/api/extra.md
+message: section "api" is reserved for the ingested module READMEs