core: the last seven missing-portable vars — coverage gap closed (jolt-brh)

The dashboard's missing-portable category is now EMPTY (was 35 when the issue
was filed; this session's io/leaf work had already landed most of them).
The final seven:

- extenders — ctx-capturing clojure.core fn over the protocol type-registry:
  the type-tags implementing a protocol, as symbols; nil when none
- find-keyword — keyword: jolt keywords have no intern table, so it always
  finds (babashka makes the same call)
- inst-ms* — the raw Inst method; one inst representation, so = inst-ms
- read+string — over the 50-io readers, which now expose :buf and :fill-fn;
  returns [form exact-text-consumed], EOF throws or yields [eof-value ""]
  with the 3-arity, works for string AND stdin readers
- with-local-vars — fresh free-standing var cells (__local-var seam) bound as
  locals; var-get/var-set work on any cell
- with-open — canonical recursive expansion closing through the __close seam:
  a map-like value's :close fn or a host file (no .close interop here);
  nested closes run inner-first, finally runs on throw
- with-precision — body evaluates with precision/:rounding accepted and
  ignored (doubles, no BigDecimal context) — documented divergence

30 new spec rows (test/spec/missing-vars-spec.janet); coverage.md
regenerated: implemented+tested 426 -> 433, missing-portable 7 -> 0.
Gate: jpm exit 0, all tests passed.

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

@@ -809,3 +809,15 @@
                  (assoc m k (up (get m k) ks f args))
                  (assoc m k (apply f (get m k) args)))))]
     (up m ks f args)))
+
+;; --- jolt-brh: the last missing-portable vars --------------------------------
+
+;; jolt keywords have no intern table (any keyword "exists"), so find-keyword
+;; always finds — babashka makes the same call.
+(defn find-keyword
+  ([nm] (keyword nm))
+  ([ns nm] (keyword ns nm)))
+
+;; The raw Inst protocol method; jolt insts have one representation, so it is
+;; inst-ms itself.
+(defn inst-ms* [i] (inst-ms i))