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.

test/spec/transients-spec.janet

@@ -57,6 +57,10 @@
 
 # assoc! (unlike assoc) accepts an odd arg count — a missing final value is nil.
 # (A struct literal can't express an explicit nil value, so assert via contains?.)
+# KNOWN BUG (jolt-ea9k): this leniency is non-Clojure — Clojure's assoc! throws on
+# odd args, as does jolt's own plain assoc. These rows encode the buggy behavior;
+# when the seed is fixed to throw, update them to :throws. The Chez host already
+# throws (Clojure-correct), so these sit in its crash bucket until then.
 (defspec "transient / assoc! odd args"
   ["odd arg key present"  "true"
    "(contains? (persistent! (assoc! (transient {}) :a 1 :b)) :b)"]