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/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}}"])