Native record representation + inline nil?/some? (#222)

Records were a jrec holding an alist of (kw . val) conses: ~113B/node, built
fresh per construction, field reads a list scan. Replace that with a shared
per-type descriptor (tag + field keywords + an eq?-keyed keyword->index table)
plus a flat per-instance value vector and an extension map for any non-field
keys assoc'd on (jolt-nil when there are none). Construction now allocates one
vector instead of a cons chain and a field read is an index lookup. binary-trees
construction allocation drops 2.085GB -> 1.19GB.

That alone barely moved binary-trees wall-time: profiling showed the read loop,
not allocation, dominates, and the read loop's own allocation came from (nil? l)
lowering to (jolt-invoke (var-deref "clojure.core" "nil?") l), which conses its
args every call. Add nil?/some? to the backend native-op table so they inline to
jolt-nil?/jolt-some? (and drop the truthy wrapper, like the other predicates).
check-tree's read loop goes from 1.476GB allocated to zero; binary-trees 18.9x
-> 9.7x vs JVM. The remaining gap is the field-read dispatch chain (jolt-c3mw).

Two JVM divergences fixed along the way, both certified:
- dissoc of a declared field downgrades a record to a plain map (was kept as a
  record); an extension key still drops cleanly.
- map->R keeps extension keys (was dropping anything outside the declared basis).

16 new corpus rows pin assoc/dissoc/count/keys/seq/=/hash/extension-field
behavior against JVM Clojure.

Co-authored-by: Yogthos <yogthos@gmail.com>

test/chez/corpus.edn

@@ -3190,4 +3190,20 @@
   {:suite "regex / literal value" :label "re-matches on a literal" :expected "\"aaa\"" :actual "(re-matches #\"a+\" \"aaa\")"}
   {:suite "regex / literal value" :label "quoted regex round-trips" :expected "\"#\\\"a.c\\\"\"" :actual "(pr-str (quote #\"a.c\"))"}
   {:suite "regex / literal value" :label "extend-protocol to Pattern dispatches" :expected "[:pattern :other]" :actual "(do (defprotocol Tg (tg [_])) (extend-protocol Tg java.util.regex.Pattern (tg [_] :pattern) Object (tg [_] :other)) [(tg #\"x\") (tg 1)])"}
+  {:suite "records / representation" :label "assoc a declared field keeps the record" :expected "[true 9 2]" :actual "(do (defrecord R [a b]) (let [x (assoc (->R 1 2) :a 9)] [(record? x) (:a x) (:b x)]))"}
+  {:suite "records / representation" :label "assoc an extension field keeps the record" :expected "[true 3 1]" :actual "(do (defrecord R [a b]) (let [x (assoc (->R 1 2) :c 3)] [(record? x) (:c x) (:a x)]))"}
+  {:suite "records / representation" :label "dissoc an extension field keeps the record" :expected "[true nil 1]" :actual "(do (defrecord R [a b]) (let [x (-> (->R 1 2) (assoc :c 3) (dissoc :c))] [(record? x) (:c x) (:a x)]))"}
+  {:suite "records / representation" :label "dissoc a declared field downgrades to a map" :expected "[false true 2 false]" :actual "(do (defrecord R [a b]) (let [m (dissoc (->R 1 2) :a)] [(record? m) (map? m) (:b m) (contains? m :a)]))"}
+  {:suite "records / representation" :label "count includes extension fields" :expected "[2 3]" :actual "(do (defrecord R [a b]) [(count (->R 1 2)) (count (assoc (->R 1 2) :c 3))])"}
+  {:suite "records / representation" :label "keys are declared then extension order" :expected "[[:a :b] [:a :b :c]]" :actual "(do (defrecord R [a b]) [(vec (keys (->R 1 2))) (vec (keys (assoc (->R 1 2) :c 3)))])"}
+  {:suite "records / representation" :label "a nil field value is present, not absent" :expected "[nil true :d]" :actual "(do (defrecord R [a b]) (let [n (->R nil 2)] [(:a n) (contains? n :a) (get (->R 1 2) :zzz :d)]))"}
+  {:suite "records / representation" :label "seq yields field map-entries" :expected "[[:a 1] [:b 2]]" :actual "(do (defrecord R [a b]) (vec (map (fn [e] [(key e) (val e)]) (->R 1 2))))"}
+  {:suite "records / representation" :label "conj a map-entry vector assocs it" :expected "3" :actual "(do (defrecord R [a b]) (:c (conj (->R 1 2) [:c 3])))"}
+  {:suite "records / representation" :label "= after assoc of an unchanged field" :expected "true" :actual "(do (defrecord R [a b]) (= (assoc (->R 1 2) :a 1) (->R 1 2)))"}
+  {:suite "records / representation" :label "= compares extension fields" :expected "[true false]" :actual "(do (defrecord R [a b]) [(= (assoc (->R 1 2) :c 9) (assoc (->R 1 2) :c 9)) (= (assoc (->R 1 2) :c 9) (->R 1 2))])"}
+  {:suite "records / representation" :label "assoc then dissoc an extension field = original" :expected "true" :actual "(do (defrecord R [a b]) (= (-> (->R 1 2) (assoc :c 3) (dissoc :c)) (->R 1 2)))"}
+  {:suite "records / representation" :label "map-> keeps extension keys" :expected "[true 3 [:a :b :c]]" :actual "(do (defrecord R [a b]) (let [m (map->R {:a 1 :b 2 :c 3})] [(record? m) (:c m) (vec (keys m))]))"}
+  {:suite "records / representation" :label "record as a map key" :expected ":v" :actual "(do (defrecord R [a]) (get {(->R 1) :v} (->R 1)))"}
+  {:suite "records / representation" :label "records dedup in a set by value" :expected "2" :actual "(do (defrecord R [a]) (count (into #{} [(->R 1) (->R 1) (->R 2)])))"}
+  {:suite "records / representation" :label "nested record field reads" :expected "[2 1]" :actual "(do (defrecord N [l r v]) (let [t (->N (->N nil nil 1) nil 2)] [(:v t) (:v (:l t))]))"}
 ]