More library-compat fixes from porting the examples (markdown, malli)

Reader / loader:
- #?@ splicing reader conditionals now actually splice the matched collection's
  items into the enclosing sequence; the splice flag was read but ignored, so a
  binding vector like [a #?@(:clj [b (.foo b)])] lost its alignment.
- the file loader reads by position and skips a top-level form that reads as
  nothing (a :cljs-only #?, a #_ discard, a trailing comment) instead of
  treating it as EOF — which silently dropped the rest of a large .cljc file.
- jolt's reader feature set now includes :clj (was {:jolt :default}). jolt is a
  Clojure/JVM-compatible host that emulates clojure.lang.* and java.* interop,
  so it reads the :clj branch of a .cljc library, not :cljs. This also lets four
  more reader-conditional corpus cases pass (floor 2726 -> 2730).

Backend:
- munge-name escapes ' (prime) -> _PRIME_; a Clojure symbol like f' otherwise
  emitted a bare ' into Scheme, which is the quote reader macro and unbalanced
  the output.

Host shims:
- clojure.java.io/writer (pass through a StringWriter, file-back a path) and a
  readLine on the string reader, so line-seq over (io/reader …) works (markdown).

A better "unsupported destructuring pattern: <pat>" error message.

host/chez/seed/image.ss

@@ -131,7 +131,7 @@
 (guard (e (#t #f))
   (def-var-with-meta! "jolt.backend-scheme" "scheme-reserved" (let* ((_o$1341 "if") (_o$1342 "begin") (_o$1343 "lambda") (_o$1344 "let") (_o$1345 "let*") (_o$1346 "letrec") (_o$1347 "letrec*") (_o$1348 "quote") (_o$1349 "quasiquote") (_o$1350 "unquote") (_o$1351 "set!") (_o$1352 "define") (_o$1353 "define-syntax") (_o$1354 "cond") (_o$1355 "case") (_o$1356 "when") (_o$1357 "unless") (_o$1358 "and") (_o$1359 "or") (_o$1360 "do") (_o$1361 "else") (_o$1362 "guard") (_o$1363 "parameterize") (_o$1364 "delay") (_o$1365 "values")) (jolt-hash-set _o$1341 _o$1342 _o$1343 _o$1344 _o$1345 _o$1346 _o$1347 _o$1348 _o$1349 _o$1350 _o$1351 _o$1352 _o$1353 _o$1354 _o$1355 _o$1356 _o$1357 _o$1358 _o$1359 _o$1360 _o$1361 _o$1362 _o$1363 _o$1364 _o$1365)) (jolt-hash-map (keyword #f "private") #t)))
 (guard (e (#t #f))
-  (def-var! "jolt.backend-scheme" "munge-name" (letrec ((munge-name (lambda (s) (let fnrec1366 ((s s)) (let* ((s (jolt-invoke (var-deref "clojure.string" "replace") s "#" "_"))) (if (jolt-contains? (var-deref "jolt.backend-scheme" "scheme-reserved") s) (jolt-invoke (var-deref "clojure.core" "str") "_" s) s)))))) munge-name)))
+  (def-var! "jolt.backend-scheme" "munge-name" (letrec ((munge-name (lambda (s) (let fnrec1366 ((s s)) (let* ((s (jolt-invoke (var-deref "clojure.string" "replace") (jolt-invoke (var-deref "clojure.string" "replace") s "#" "_") "'" "_PRIME_"))) (if (jolt-contains? (var-deref "jolt.backend-scheme" "scheme-reserved") s) (jolt-invoke (var-deref "clojure.core" "str") "_" s) s)))))) munge-name)))
 (guard (e (#t #f))
   (declare-var! "jolt.backend-scheme" "emit"))
 (guard (e (#t #f))