Fix record field-reorder redefine (jolt-wf4) and builtin-shadowing local names (jolt-fjb1) (#124)

* Don't direct-link a var redefined earlier in the same unit (jolt-wf4)

defrecord/deftype expands to (do (def R (make-deftype-ctor ...)) (def ->R R) ...),
so the ->R alias references R within one compiled unit. Under direct-link a var
ref embeds the cell's root as a compile-time constant, but on a redefine R's old
root is still in place when that unit compiles — the (def R new) sibling hasn't
run yet — so ->R sealed to the stale pre-redef ctor. (defrecord R [x a])
(defrecord R [a x]) (:a (->R 10 20)) read the old [x a] layout and returned 20.

Track the vars a unit (re)defines and force their later in-unit references to the
live indirect deref. The cell is registered only after its own init is emitted,
so a recursive self-reference inside the init still direct-links (it runs after
the def completes); only sibling references after the def go indirect.

* Emit Janet's `in` as a value so a user local can't shadow it (jolt-fjb1)

The back end emits `in` to deref var cells ((in cell :root)) and index
shape-recs. It emitted the bare symbol, so a user local named `in` shadowed
Janet's builtin in the surrounding scope and the generated cell-deref called the
user's value as a function — "<table> called with 2 arguments, possibly expected
1". malli's explainer binds [value in acc], so m/explain hit this on every
schema (m/validate was unaffected — its path doesn't bind `in`).

Embed `in`'s function value at the emit sites (as jolt-call/core-get already
are); a value in head position can't be shadowed. Fixes m/explain on malli
(loaded with JOLT_FEATURES=clj so its .cljc reader-conditionals resolve).

---------

Co-authored-by: Yogthos <yogthos@gmail.com>

test/spec/functions-spec.janet

@@ -11,7 +11,13 @@
   ["variadic with fixed" "[1 [2 3]]" "(do (defn f [a & xs] [a xs]) (f 1 2 3))"]
   ["closure captures"   "8"      "(do (defn adder [n] (fn [x] (+ x n))) ((adder 5) 3))"]
   ["recursion"          "120"    "(do (defn fact [n] (if (< n 2) 1 (* n (fact (dec n))))) (fact 5))"]
-  ["named fn self-ref"  "120"    "((fn fact [n] (if (< n 2) 1 (* n (fact (dec n))))) 5)"])
+  ["named fn self-ref"  "120"    "((fn fact [n] (if (< n 2) 1 (* n (fact (dec n))))) 5)"]
+  # A param whose name collides with a Janet builtin the back end emits in
+  # generated code (here `in`, used to deref vars / index shape-recs) must not
+  # shadow it — malli's explainer binds `[value in acc]` (jolt-fjb1).
+  ["param named `in`"   "1"       "((fn [in] (first in)) [1 2 3])"]
+  ["param `in` via core fn" "3"   "((fn [in] (count in)) [1 2 3])"]
+  ["local `in` in let body" "2"   "(let [in [2 3]] (first in))"])
 
 (defspec "functions / application"
   ["apply"              "6"      "(apply + [1 2 3])"]