JVM-semantics fixes and small cleanups

- take-last / drop-last return seqs, not vectors: take-last wraps in seq; drop-last
  is the JVM (map (fn [x _] x) coll (drop n coll)) form (lazy, () when empty).
- cycle is lazy ((lazy-seq (concat coll (cycle coll)))) so it no longer counts its
  argument and terminates on a lazy/infinite input.
- fold's foldable-call catch uses :default, matching the rest of jolt-core and
  also catching a raw host condition from a folding primitive.
- alts! rejects non-channel ports with a clear error (put specs / :default are
  unsupported) instead of crashing inside ac-poll!.
- Misc: drop the unreachable second getCause clause; jolt-nth on a string raises
  'nth "index out of bounds" like the vector branch; name the inline fixpoint cap;
  bld-sh-capture rejoins lines with newlines; clarify a couple of comments.

host/chez/records.ss

@@ -174,7 +174,9 @@
                  (keyword #f "name") (jolt-symbol jolt-nil name-str)
                  (keyword #f "methods") methods))
 
-;; register-protocol-methods!: a no-op for Chez dispatch.
+;; register-protocol-methods!: intentional no-op. Chez dispatches a protocol method
+;; by the receiver's type tag at call time, so there is no method table to register;
+;; this binding exists only because defprotocol-emitted code calls it.
 (define (register-protocol-methods! proto-name method-names) jolt-nil)
 
 ;; register-method: extend-type/extend register an impl. Host type names keep a
@@ -329,7 +331,7 @@
              ((string=? method-name "toString") (condition->message-string obj))
              ((string=? method-name "getCause") jolt-nil)
              ;; java.sql.SQLException chaining — jolt errors don't chain (nil).
-             ((or (string=? method-name "getNextException") (string=? method-name "getCause")) jolt-nil)
+             ((string=? method-name "getNextException") jolt-nil)
              ((string=? method-name "getStackTrace") (jolt-vector))
              ((string=? method-name "printStackTrace") jolt-nil)
              (else (error #f (string-append "No method " method-name " on Throwable")))))