Fix five bugs surfaced by commonmark-app (jolt-3xur, dl4s, ik3a, w2wf, xtss) (#158)

* Fix four bugs surfaced by the commonmark-app example

- regex: bounded quantifiers {n,m} no longer expand exponentially. The
  desugaring inlined the continuation per optional level, doubling it each
  time, so {0,61} built a PEG the compiler expanded to ~2^61 nodes and hung.
  Each level is now its own grammar rule referenced by name (jolt-3xur).

- strings are a seqable of chars for vec/set/into, matching seq. They went
  through realize-for-iteration, which had no string case, so into/set got
  raw bytes (code points) and set threw; vec used string/from-bytes (1-char
  strings). realize-for-iteration now maps make-char over the bytes, and
  core-vec matches (jolt-dl4s).

- clojure.string/split takes Java Pattern.split limit semantics: negative
  keeps trailing empties, 0/omitted drops them (a no-match result stays
  [input]), positive caps the part count with the remainder unsplit. It used
  to delegate the limit to take, so a negative limit returned [] and the
  2-arg form never trimmed trailing empties (jolt-ik3a).

- System/exit is registered (maps to os/exit), so (System/exit n) works
  (jolt-w2wf).

* regex: single-digit backreferences \1..\9 (jolt-xtss)

\1..\9 now match the text captured by the corresponding group, so
patterns like ([-*_])\1\1 or (\w+) \1 work. The parser records which
groups are referenced; a referenced group additionally captures its text
under a tag and the backref compiles to a PEG (backmatch). Only referenced
groups change — they match possessively (the CPS-over-possessive-PEG engine
can't backtrack into a tagged capture), so backtracking back into a
backreferenced group isn't supported (rare). Unreferenced groups keep full
backtracking and position-based result capture unchanged.

---------

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

test/spec/host-interop-spec.janet

@@ -159,6 +159,8 @@
    "[(Boolean/parseBoolean \"true\") (Boolean/parseBoolean \"false\") (Boolean/parseBoolean \"yes\")]"]
   ["System/getenv is a map" "true"
    "(string? (get (System/getenv) \"HOME\"))"]
+  # registered, so (System/exit n) resolves; actual exit verified via subprocess
+  ["System/exit resolves" "true" "(fn? System/exit)"]
   # NOT every? alone — it held vacuously while seq over a raw host table
   # yielded nothing, hiding that read-system-env came back empty
   ["getenv entries destructure (non-empty)" "true"