Chez Phase 1 (increment 3p): misc seq/regex gaps + bug tracking

jolt-y1zq tail:
- 0-arg (conj) -> [] and 0-arg (conj!) -> a fresh transient vector
- nth sees through a transient (like get/count/contains?)
- irregex \p{...}/\P{...} property classes translate to the seed's ASCII char
  classes (regex.ss): \p{L} -> [a-zA-Z] + non-ASCII codepoints (the seed counts
  UTF-8 high bytes as letters), \p{N} -> [0-9], \p{Ps} -> [([{], etc. The
  translator tracks [...] nesting so a \p{} inside a class emits its content, not
  a nested class.

Two pre-existing bugs found and filed (tracked, not replicated):
- jolt-x0os: the Chez emitter mangles non-ASCII string literals into invalid Chez
  hex escapes (so p{L} utf-8 crashes on the input string, not the regex).
- jolt-ea9k: the seed's transient assoc! accepts odd args and assigns nil to the
  trailing key (non-Clojure; plain assoc throws). The Chez host throws
  (Clojure-correct); the 4 spec rows encoding the leniency are flagged in
  transients-spec.janet pending the seed fix.

Parity 1493 -> 1506/2497, 0 new divergences. emit-test 291/291.

host/chez/collections.ss

@@ -202,11 +202,14 @@
                 (pmap-assoc coll (pvec-nth-d x 0 jolt-nil) (pvec-nth-d x 1 jolt-nil)))
                (else (error 'conj "conj on a map expects a [k v] pair or a map"))))
         (else (error 'conj "unsupported collection"))))
-;; (conj nil a b ...) builds a list in Clojure, conj prepending -> (b a).
-(define (jolt-conj coll . xs)
-  (if (jolt-nil? coll)
-      (fold-left jolt-conj1 jolt-empty-list xs)
-      (fold-left jolt-conj1 coll xs)))
+;; (conj) -> []; (conj nil a b ...) builds a list (conj prepending -> (b a)).
+(define (jolt-conj . args)
+  (if (null? args)
+      (jolt-vector)
+      (let ((coll (car args)) (xs (cdr args)))
+        (if (jolt-nil? coll)
+            (fold-left jolt-conj1 jolt-empty-list xs)
+            (fold-left jolt-conj1 coll xs)))))
 
 (define jolt-get
   (case-lambda