feat: SCI bootstrap loads completely (422/422) — fix with-meta + select-keys

The last two keystone forms in namespaces.cljc now load, so the entire SCI
source in the harness's load order evaluates with ZERO failures (was 420/2).

Two real bugs fixed to get there:
- with-meta crashed on non-collections: (with-meta a-fn {...}) did (keys fn) ->
  'expected iterable, got function'. SCI marks core fns private this way. Now
  returns functions/scalars unchanged (Jolt can't attach meta to a raw fn).
- select-keys ignored a vector key-list: (each k ks) iterated the pvec's table
  instead of its elements, so (select-keys m [:a]) returned {}. Now realizes ks.

Plus: add clojure.set/join (the only missing clojure.set fn), assert 0 failures
in test-load-sci so it's a real regression guard, drop debug instrumentation.

Jolt now fully loads SCI's reader-independent implementation: every pure-Clojure
module, the entire clojure.core aggregation map, and the clojure.* namespace
wiring. conformance 218/218, features 78/78, jank 120.

src/jolt/clojure/set.clj

@@ -40,6 +40,33 @@
   [xrel ks]
   (reduce (fn [m x] (let [ik (select-keys x ks)] (assoc m ik (conj (get m ik #{}) x)))) {} xrel))
 
+(defn join
+  "When passed 2 rels, returns the rel corresponding to the natural join.
+  When passed an additional keymap, joins on the corresponding keys."
+  ([xrel yrel]
+   (if (and (seq xrel) (seq yrel))
+     (let [ks (intersection (set (keys (first xrel))) (set (keys (first yrel))))
+           [r s] (if (<= (count xrel) (count yrel)) [xrel yrel] [yrel xrel])
+           idx (index r ks)]
+       (reduce (fn [ret x]
+                 (let [found (idx (select-keys x ks))]
+                   (if found
+                     (reduce (fn [acc y] (conj acc (merge y x))) ret found)
+                     ret)))
+               #{} s))
+     #{}))
+  ([xrel yrel km]
+   (let [[r s k] (if (<= (count xrel) (count yrel))
+                   [xrel yrel (map-invert km)]
+                   [yrel xrel km])
+         idx (index r (vals k))]
+     (reduce (fn [ret x]
+               (let [found (idx (rename-keys (select-keys x (keys k)) k))]
+                 (if found
+                   (reduce (fn [acc y] (conj acc (merge y x))) ret found)
+                   ret)))
+             #{} s))))
+
 (defn subset?
   [set1 set2]
   (and (<= (count set1) (count set2))

src/jolt/core.janet

@@ -494,7 +494,7 @@
 
 (defn core-select-keys [m ks]
   (var result @{})
-  (each k ks
+  (each k (realize-for-iteration ks)
     (let [v (core-get m k)]
       (if (not (nil? v)) (put result k v))))
   (if (struct? m) (table/to-struct result) result))
@@ -2237,15 +2237,21 @@
     (put prefs dispatch-val-a dispatch-val-b) mm-var))
 
 (defn core-with-meta [obj meta]
-  (var new-obj @{})
-  (each k (keys obj)
-    (put new-obj k (get obj k)))
-  # table/setproto requires a table, convert struct meta to table
-  (var meta-tab @{})
-  (each k (keys meta) (put meta-tab k (get meta k)))
-  (table/setproto new-obj meta-tab)
-  (put new-obj :jolt/meta meta)
-  new-obj)
+  # Functions and scalars can't carry metadata in Jolt's model — return as-is
+  # rather than crashing (Clojure attaches meta only to IObj values).
+  (if (or (function? obj) (cfunction? obj) (number? obj) (boolean? obj)
+          (nil? obj) (string? obj) (keyword? obj) (buffer? obj))
+    obj
+    (do
+      (var new-obj @{})
+      (each k (keys obj)
+        (put new-obj k (get obj k)))
+      # table/setproto requires a table, convert struct meta to table
+      (var meta-tab @{})
+      (each k (keys meta) (put meta-tab k (get meta k)))
+      (table/setproto new-obj meta-tab)
+      (put new-obj :jolt/meta meta)
+      new-obj)))
 
 (defn core-var-dynamic? [v]
   (var-dynamic? v))