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.

check.sh

@@ -37,14 +37,4 @@ else
 	echo "skipped: no python3"
 fi
 
-# Differential tests against the Python these tools were ported from. Skipped
-# unless an interpreter with the dependencies is named, since most machines do
-# not have one.
-if [ -n "${CATENA_PARITY_PYTHON:-}" ]; then
-	say "parity against python"
-	cargo test -p parity --features python
-else
-	say "parity against python (skipped: set CATENA_PARITY_PYTHON to run it)"
-fi
-
 printf '\nall clear\n'