feat: java.io.File model + multimethod/assoc/defmulti fixes for migratus (jolt-hjw)

File API (jolt-hjw): io/file and (File. …) build a tagged :jolt/file value
(instance? File true) with a full method surface (isFile/isDirectory/exists/
getName/getPath/getAbsolutePath/listFiles/toPath/delete/createNewFile/…) backed
by os/ and file/. file-seq is File-aware (leaves are File values). str/slurp/spit
coerce :jolt/file to its path. ClassLoader/getSystemClassLoader + a classloader
stub whose getResource returns nil degrade migratus's classpath lookup to the
filesystem. java.nio.file Path/FileSystem/PathMatcher are shimmed just enough for
script-excluded?'s glob (recursive * / ? matcher).

Three bugs found getting migratus's migration discovery to work:
- (assoc nil k v) returned a raw janet table, not a map, so assoc-in built tables
  that count/seq rejected. Now returns an immutable map.
- methods/get-method resolved the multimethod symbol at runtime in the current
  ns, so a bare multifn ref in its defining ns saw an empty table once defmethods
  lived elsewhere. Now they take the multimethod VALUE and recover the var via a
  registry (Clojure semantics).
- defmulti now drops a leading docstring/attr-map (migratus's multimethods carry
  docstrings) instead of treating the docstring as the dispatch fn.

Conformance 335/335 x3, clojure-test-suite at baseline.

jolt-core/clojure/core/20-coll.clj

@@ -519,7 +519,11 @@
 ;; via the host dir primitives. Paths (strings), not File objects. (Lives below
 ;; tree-seq: forward references are analysis errors now — jolt-2o7.3.)
 (defn file-seq [root]
-  (tree-seq __dir? __list-dir root))
+  (if (__file? root)
+    ;; java.io.File tree: walk via the File method surface so leaves are File
+    ;; values callers can invoke .isFile/.getName/slurp on (jolt-hjw).
+    (tree-seq (fn [f] (.isDirectory f)) (fn [f] (seq (.listFiles f))) root)
+    (tree-seq __dir? __list-dir root)))
 
 ;; Canonical flatten via tree-seq: the leaves (non-sequential nodes) in order.
 ;; Flattens lists too (sequential?), matching Clojure/CLJS.