core: three bug fixes — ifn?, prefer-method dispatch, reader comments in map values

ifn? (jolt-1vx) is the canonical IFn set in the overlay: fns, keywords,
symbols, maps (sorted included), sets, vectors, and vars — NOT lists. The
seed version said true for lists and false for struct maps and vars.
Mutable-mode caveat documented (vectors and lists share the array repr
there). 13 predicate rows.

Multimethod dispatch (jolt-heo) now collects EVERY isa-matching method key
and picks the dominant one — x dominates y when prefer-method'd over it or
(isa? x y) — and two matches with no dominant is an ambiguity ERROR, as in
Clojure. It used to take whichever key the table yielded first, silently
ignoring prefer-method. The prefers store upgrades to Clojure's
{x -> set-of-dominated} shape, shared between the dispatch closure and
prefer-method-setup via the var; prefers becomes a macro over a setup fn
(the store lives on the VAR — the multifn value can't carry it, so the old
fn read {} forever). 6 multimethod rows + the conformance row updated to
the canonical shape (335x3).

The reader (jolt-ou8) kept the pending KEY when a comment or #_ sits in a
map's VALUE slot: the old code dropped both, desyncing kv pairing — the
real value became the next key and the closing brace landed in value
position ('Unmatched closing brace'). Selmer's deps.edn (a '; for
development (REPL, etc)' comment between key and value) now parses; 6
reader rows incl. nested commented maps.

Gate: jpm exit 0, conformance 335x3, all tests passed.

test/spec/multimethods-spec.janet

@@ -30,3 +30,19 @@
    "(do (defmulti classify :type :default :other) (defmethod classify :a [_] :alpha) (defmethod classify :other [_] :unknown) (classify {:type :zzz}))"]
   ["explicit :hierarchy"  "\"a\""
    "(do (def h (derive (make-hierarchy) ::dog ::animal)) (defmulti snd identity :hierarchy h) (defmethod snd ::animal [_] \"a\") (snd ::dog))"])
+
+# prefer-method breaks isa-dispatch ties; ambiguity without a preference is
+# an ERROR (jolt-heo — this used to silently take an arbitrary method).
+(defspec "multimethods / prefer-method"
+  ["preference picks the winner" ":rect"
+   "(do (derive :p/sq :p/rect) (derive :p/sq :p/shape) (defmulti pm1 identity) (defmethod pm1 :p/rect [x] :rect) (defmethod pm1 :p/shape [x] :shape) (prefer-method pm1 :p/rect :p/shape) (pm1 :p/sq))"]
+  ["reverse preference" ":shape"
+   "(do (derive :q/sq :q/rect) (derive :q/sq :q/shape) (defmulti pm2 identity) (defmethod pm2 :q/rect [x] :rect) (defmethod pm2 :q/shape [x] :shape) (prefer-method pm2 :q/shape :q/rect) (pm2 :q/sq))"]
+  ["ambiguity throws" :throws
+   "(do (derive :r/sq :r/rect) (derive :r/sq :r/shape) (defmulti pm3 identity) (defmethod pm3 :r/rect [x] :rect) (defmethod pm3 :r/shape [x] :shape) (pm3 :r/sq))"]
+  ["isa dominance needs no preference" ":child"
+   "(do (derive :s/c :s/p) (defmulti pm4 identity) (defmethod pm4 :s/c [x] :child) (defmethod pm4 :s/p [x] :parent) (pm4 :s/c))"]
+  ["prefers map shape" "true"
+   "(do (defmulti pm5 identity) (defmethod pm5 :a [x] 1) (defmethod pm5 :b [x] 2) (prefer-method pm5 :a :b) (contains? (get (prefers pm5) :a) :b))"]
+  ["exact match needs no preference" ":exact"
+   "(do (derive :t/sq :t/rect) (defmulti pm6 identity) (defmethod pm6 :t/sq [x] :exact) (defmethod pm6 :t/rect [x] :parent) (pm6 :t/sq))"])

test/spec/predicates-spec.janet

@@ -184,3 +184,19 @@
   ["int? Inf false"     "false" "(int? ##Inf)"]
   ["integer? Inf false" "false" "(integer? ##Inf)"]
   ["integer? NaN false" "false" "(integer? ##NaN)"])
+
+# ifn? is the canonical IFn set (jolt-1vx): lists are NOT IFn.
+(defspec "predicates / ifn?"
+  ["fn"        "true"  "(ifn? inc)"]
+  ["keyword"   "true"  "(ifn? :k)"]
+  ["symbol"    "true"  "(ifn? (quote s))"]
+  ["map"       "true"  "(ifn? {})"]
+  ["sorted map" "true" "(ifn? (sorted-map))"]
+  ["set"       "true"  "(ifn? #{1})"]
+  ["vector"    "true"  "(ifn? [1])"]
+  ["var"       "true"  "(ifn? (var first))"]
+  ["list NOT"  "false" "(ifn? (list 1 2))"]
+  ["lazy NOT"  "false" "(ifn? (map inc [1]))"]
+  ["string NOT" "false" "(ifn? \"s\")"]
+  ["number NOT" "false" "(ifn? 5)"]
+  ["nil NOT"   "false" "(ifn? nil)"])

test/spec/reader-syntax-spec.janet

@@ -58,3 +58,15 @@
   ["tagged literal var" "true"      "(var? #'+)"]
   ["deref sugar"        "5"         "(let [a (atom 5)] @a)"]
   ["meta sugar"         "{:t 1}"    "(meta ^{:t 1} [])"])
+
+# Comments and #_ discards in a map's VALUE slot keep the pending key
+# (jolt-ou8: dropping both desynced kv pairing — Selmer's deps.edn, with a
+# "; for development (REPL, etc)" comment between key and value, read its
+# closing brace in value position: 'Unmatched closing brace').
+(defspec "reader / comments inside maps"
+  ["comment in value slot"   "{:a 1}"  "{:a ; note\n 1}"]
+  ["comment before key"      "{:a 1}"  "{; lead\n :a 1}"]
+  ["comment between entries" "{:a 1 :b 2}" "{:a 1 ; mid\n :b 2}"]
+  ["discard in value slot"   "{:a 1}"  "{:a #_9 1}"]
+  ["comment with parens"     "{:a {:b 1}}" "{:a ; dev (REPL, etc)\n {:b 1}}"]
+  ["nested with comments"    "{:x {:y 2}}" "{:x ; outer\n {:y ; inner\n 2}}"])