interop: String/valueOf static + String is a CharSequence

Two interop gaps surfaced bringing up hiccup and malli:
- String/valueOf(Object): hiccup's compiler stringifies attribute values with
  (String/valueOf (or arg "")). Added the static — "null" for nil, else core-str.
- (instance? CharSequence s) returned false for a string; String implements
  CharSequence, and malli's :re validator gates on it before matching, so :re
  schemas always failed. instance-check now answers true for strings.

src/jolt/eval_runtime.janet

@@ -707,6 +707,10 @@
           "Double" (number? val)
           "String" (string? val)
           "java.lang.String" (string? val)
+          # String implements CharSequence — malli's :re validator gates on
+          # (instance? CharSequence x) before matching (jolt-ltwk).
+          "CharSequence" (string? val)
+          "java.lang.CharSequence" (string? val)
           "Boolean" (or (= true val) (= false val))
           "Keyword" (keyword? val)
           # regex patterns (cuerdas-style (instance? Pattern x) checks)

src/jolt/interop/host_io.janet

@@ -288,6 +288,12 @@
   # (String. bytes) / (String. bytes charset): UTF-8 bytes to string.
   (each nm ["String" "java.lang.String"]
     (register-class-ctor! nm (fn [x &opt charset] (string x))))
+  # String statics. valueOf(Object) yields "null" for nil, else toString —
+  # hiccup's compiler emits (String/valueOf (or arg "")) to stringify attribute
+  # values; core-str gives jolt's str semantics for keywords/numbers/etc.
+  (each nm ["String" "java.lang.String"]
+    (register-class-statics! nm
+      @{"valueOf" (fn [x &opt off cnt] (if (nil? x) "null" (core-str x)))}))
   # java.net.URL: enough for selmer's template cache — file: URLs only.
   # A protocol-less spec throws (selmer catches MalformedURLException and
   # prepends file:///), and getPath hands back a stat-able filesystem path.

test/spec/host-interop-spec.janet

@@ -79,6 +79,14 @@
   [".charAt"        "\\b"     "(.charAt \"abc\" 1)"]
   [".equalsIgnoreCase" "true" "(.equalsIgnoreCase \"AbC\" \"aBc\")"]
   ["Long/MAX_VALUE" "true"    "(pos? Long/MAX_VALUE)"]
+  # String/valueOf static (jolt-nkx): hiccup stringifies attribute values with it.
+  ["String/valueOf num"  "\"42\""   "(String/valueOf 42)"]
+  ["String/valueOf str"  "\"hi\""   "(String/valueOf \"hi\")"]
+  ["String/valueOf kw"   "\":k\""   "(String/valueOf :k)"]
+  ["String/valueOf nil"  "\"null\"" "(String/valueOf nil)"]
+  # String implements CharSequence (malli's :re gates on it, jolt-ltwk).
+  ["instance? CharSequence" "true"  "(instance? CharSequence \"aaa\")"]
+  ["instance? CharSequence non-str" "false" "(instance? CharSequence 42)"]
   ["unsupported method throws" :throws "(.frobnicate \"abc\")"])
 
 # java.time shims (jolt-ea7): epoch-ms backed values + a DateTimeFormatter