Close portable clojure.core gaps: re-groups, letfn, REPL + dynamic vars

Spec coverage dashboard had 6 missing-portable and 24 dynamic-var entries. The
portable ones are now implemented (missing-portable -> 0, dynamic-var -> 14):

- Stateful matcher: re-matcher now returns a real mutable Matcher; re-find over
  it steps through matches and re-groups returns the last match's groups (was an
  inert tagged map). Closes re-groups.
- letfn is interned as a clojure.core var so (resolve 'letfn) matches the JVM. It
  stays a special form (the value is never invoked, not marked a macro).
- *1 *2 *3 *e interned (nil outside a REPL).
- Portable dynamic vars whose default already matches jolt's behaviour:
  *read-eval* *print-dup* *print-namespace-maps* *flush-on-newline*
  *compile-files* *math-context* *command-line-args* *file*.

The remaining 14 dynamic-var entries are host-internal (compile-path,
compiler-options, fn-loader, reader-resolver, repl, source-path, ...) or deferred
pending printer/reader support (*print-length* *print-level*
*default-data-reader-fn*). Corpus rows added for each closed gap; coverage.md
regenerated.

host/chez/dynamic-var-defaults.ss

@@ -28,3 +28,35 @@
 
 ;; *print-meta* — when true, pr prints metadata with a ^ prefix; default false.
 (def-var! "clojure.core" "*print-meta*" #f)
+
+;; Portable clojure.core dynamic vars whose DEFAULT already matches jolt's
+;; behaviour, so exposing them is sound (resolve/binding work, reads return the
+;; right value) — not a silent divergence. The ones that change observable output
+;; if set non-default (*print-length* / *print-level* / *default-data-reader-fn*)
+;; are deliberately NOT interned until the printer/reader honour them.
+;;
+;; *read-eval* — gates #=() read-eval. jolt's reader has no #=, so it reads true
+;; (no eval-on-read happens regardless); a lib can (binding [*read-eval* false] …).
+(def-var! "clojure.core" "*read-eval*" #t)
+;; *print-dup* — gates print-dup (a multimethod that exists); default false.
+(def-var! "clojure.core" "*print-dup*" #f)
+;; *print-namespace-maps* — jolt never prints the #:ns{…} map shorthand, so the
+;; var reads false (accurate); settable for code that toggles it.
+(def-var! "clojure.core" "*print-namespace-maps*" #f)
+;; *flush-on-newline* — jolt flushes line output; default true.
+(def-var! "clojure.core" "*flush-on-newline*" #t)
+;; *compile-files* — jolt has no separate compile phase that emits .class files.
+(def-var! "clojure.core" "*compile-files*" #f)
+;; *math-context* — BigDecimal rounding context; nil = unlimited, jolt's default.
+(def-var! "clojure.core" "*math-context*" jolt-nil)
+;; *command-line-args* — the args after the script/-main; nil outside a -m run.
+(def-var! "clojure.core" "*command-line-args*" jolt-nil)
+;; *file* — the source file being loaded; "NO_SOURCE_PATH" when none, like the JVM.
+(def-var! "clojure.core" "*file*" "NO_SOURCE_PATH")
+
+;; REPL result/exception history. Bound by the REPL after each evaluation; nil
+;; outside a REPL, which is what reading them returns here.
+(def-var! "clojure.core" "*1" jolt-nil)
+(def-var! "clojure.core" "*2" jolt-nil)
+(def-var! "clojure.core" "*3" jolt-nil)
+(def-var! "clojure.core" "*e" jolt-nil)