compiler: IFn collections held in a var/local are callable (jolt-vh5)

The back end emitted a direct Janet call (f args) for every :var/:local callee,
but Janet calling a pvec-table does get (no integer key -> nil), a keyword does
the wrong thing, etc. — so (let [v [1 2 3]] (v 0)), a keyword/meta-vector in a
binding, all returned nil in compile mode (interpret was fine). Now a direct call
is emitted only when the callee is provably a function: :fn / :host always, and a
:var whose current root is a function; everything else routes through jolt-call,
which dispatches IFn correctly (function fast-path first). User/core fn calls stay
direct, so no hot-path regression — fixpoint stage1==stage2==stage3 holds.
Conformance 218/218 all modes; full suite green. Spec cases added.

src/jolt/backend.janet

@@ -156,8 +156,21 @@
       ['do ['var s nil] ['set s body] s])
     body))
 
-(defn- direct-call? [fnode]
-  (case (fnode :op) :var true :local true :fn true :host true false))
+# A direct Janet call (f args) is only correct when the callee is definitely a
+# function: Janet calling a pvec/keyword/etc. does get (or the wrong thing), not
+# IFn dispatch. So only emit a direct call for :fn / :host (always functions) and
+# a :var whose CURRENT root is a function (the common user/core-fn case). A :var
+# holding an IFn COLLECTION (vector/keyword/set used as a fn) or a :local of
+# unknown value falls through to jolt-call, which dispatches IFn correctly
+# (function fast-path first). Trade-off, like direct-linking: a fn-var redefined
+# to a collection after this call was compiled would still emit a direct call.
+(defn- direct-call? [ctx fnode]
+  (case (fnode :op)
+    :fn true
+    :host true
+    :var (let [r (get (cell-for ctx (fnode :ns) (fnode :name)) :root)]
+           (or (function? r) (cfunction? r)))
+    false))
 
 # Hot primitives emitted as native Janet ops (host-specific optimization): a
 # call to clojure.core/+ etc. becomes (+ …) rather than a var deref + variadic
@@ -188,7 +201,7 @@
           '++ ['+ (in args 0) 1]
           '-- ['- (in args 0) 1]
           (tuple nop ;args))
-    (direct-call? (node :fn)) (tuple (emit ctx (node :fn)) ;args)
+    (direct-call? ctx (node :fn)) (tuple (emit ctx (node :fn)) ;args)
     (tuple jolt-call (emit ctx (node :fn)) ;args)))
 
 (defn- emit-vector [ctx node]

test/spec/vectors-spec.janet

@@ -24,7 +24,12 @@
   ["count"                  "3"         "(count [1 2 3])"]
   ["contains? index"        "true"      "(contains? [:a :b] 1)"]
   ["contains? past end"     "false"     "(contains? [:a] 3)"]
-  ["vector as fn"           ":b"        "([:a :b :c] 1)"])
+  ["vector as fn"           ":b"        "([:a :b :c] 1)"]
+  # An IFn collection held in a binding (not just a literal) must dispatch as IFn,
+  # not as a host call: applies to vectors, keywords, and meta-bearing vectors.
+  ["vector-in-local as fn"  "20"        "(let [v [10 20 30]] (v 1))"]
+  ["keyword-in-local as fn" "7"         "(let [k :a] (k {:a 7}))"]
+  ["meta vector as fn"      "10"        "((with-meta [10 20] {:k 1}) 0)"])
 
 (defspec "vector / update (persistent)"
   ["conj appends"           "[1 2 3]"   "(conj [1 2] 3)"]