rand-nth follows the reference shape; refresh doc counts and the corpus floor

rand-nth vec'd its argument, so (rand-nth nil) hit index-out-of-bounds
through the empty vector where the reference's (nth coll (rand-int (count
coll))) returns nil (and a set throws) — the last genuinely-fixable row in
the suite baseline; rand-nth is fully clean now. Corpus/README counts
updated to the current ~3570 rows, the run-corpus regression floor raised
from 2730 to 3390 to match current parity, and the stale traceability line
dropped.

test/chez/README.md

@@ -7,7 +7,7 @@ test` from the repo root.
 
 ## The spec corpus
 
-`corpus.edn` is the contract: ~2920 rows `{:suite :label :expected :actual}`, with
+`corpus.edn` is the contract: ~3570 rows `{:suite :label :expected :actual :portability}`, with
 `:expected` sourced from reference JVM Clojure by `test/conformance/regen-corpus.clj`.
 It is frozen (the canonical source) — add or change cases here, then re-source the
 answers with `regen-corpus.clj` and re-certify with `test/conformance/certify.clj`.
@@ -22,7 +22,7 @@ answers with `regen-corpus.clj` and re-certify with `test/conformance/certify.cl
 
       chez --script host/chez/run-corpus.ss
       JOLT_CORPUS_LIMIT=200 …            # every-Nth stride, fast iteration
-      JOLT_CHEZ_ZJ_FLOOR=N …            # override the floor (default 2678)
+      JOLT_CHEZ_ZJ_FLOOR=N …            # override the floor (see run-corpus.ss)
 
 - `run-unit.ss` — host-specific unit cases (`test/chez/unit.edn`) that aren't in the
   JVM-portable corpus: dot-forms, java statics, io, reader, walk, vars/namespaces,

test/chez/cts-known-failures.txt

@@ -23,7 +23,6 @@ clojure.core-test.plus 11 0
 clojure.core-test.plus-squote 11 0
 clojure.core-test.pos-int-qmark 1 0
 clojure.core-test.quot 25 0
-clojure.core-test.rand-nth 0 1
 clojure.core-test.realized-qmark 1 0
 clojure.core-test.rem 16 0
 clojure.core-test.remove-watch 0 1