drop the parity harness, keep what it argued

The harness compared these tools against the code they replaced — Python
imported from book-pipeline, podcast-pipeline and knowledge-mcp, and
JavaScript copied out of the podcast media server and indexer, each pinned to
a commit. All of it has since been deleted, so the live half cannot run and
the golden half pins answers against a source nobody can consult.

What was worth keeping was never the comparison. `KNOWN.md` argued, at
length, why a dozen of these tools deliberately do something a reader would
not expect: why pdfgutter measures the crop box throughout, why avplan finds
the encoder rather than reading one out of the environment, why mdscrub will
not manufacture a page boundary. Those arguments are about this code and
stand on their own, so they are now `docs/decisions.md`, rewritten to say
what each tool does and why rather than what it does differently.

Binding coverage is unaffected: the bindings carry 24 tests of their own, a
node smoke test against the built package, and a typecheck.

tools/check-links.py

@@ -42,7 +42,7 @@ def anchors(text: str) -> set[str]:
 
 def main() -> int:
     root = pathlib.Path(".").resolve()
-    # Fixtures are not documentation: a parity case is deliberately malformed
+    # Fixtures are not documentation: a test case is deliberately malformed
     # markdown, and a link in one is part of what is being tested.
     skip = {"target", "cases", "golden"}
     files = sorted(