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/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)"])