Conformance: throwable chaining, URL ctor/getProtocol, ClassLoader/Thread shims

Surfaced running the DB libraries (migratus) on the jolt db library:
- java.sql.SQLException .getNextException / .getStackTrace / .printStackTrace on
  jolt throwables (conditions + ex-info) return nil/empty, so a library walking
  the exception chain doesn't crash.
- java.net.URL ctor + .getProtocol (file/http), alongside the existing url shim.
- A generic java.lang.ClassLoader: getSystemClassLoader / a thread's
  contextClassLoader resolve a named resource against the source roots (the same
  model as clojure.java.io/resource) — file: URL or nil. Thread/currentThread.
  These are generic host capabilities, not DB-specific.

The jolt-lang/db next.jdbc surface now runs migratus far enough to connect, build
the migrations table, and discover migrations; migratus's remaining dependency is
java.nio.file (FileSystems/Path/PathMatcher glob), a JVM filesystem API kept out
of core.

host/chez/dot-forms.ss

@@ -57,6 +57,9 @@
                (jolt-get obj jolt-kw-message jolt-nil)
                (jolt-str-render-one obj))))
     ((string=? name "getCause")  (list (jolt-get obj jolt-kw-cause jolt-nil)))
+    ;; java.sql.SQLException chaining — ex-info / host throwables don't chain.
+    ((string=? name "getNextException") (list jolt-nil))
+    ((string=? name "getStackTrace") (list (jolt-vector)))
     ((string=? name "toString")  (list (jolt-str-render-one obj)))
     ((string=? name "hashCode")  (list (jolt-hash obj)))
     ((string=? name "equals")    (list (if (jolt= obj (car args)) #t #f)))