core: #inst instant values + syntax-quote literal collapse (spec 2.3/2.4)

#inst (jolt-rnh): an instant is an immutable tagged struct
{:jolt/type :jolt/inst :ms <epoch-millis>} — equality and map-key hashing by
INSTANT, so different offsets denoting the same moment are =. The reader
parses RFC3339 with Clojure's partial-timestamp defaults (#inst "2020" is
2020-01-01T00:00:00.000Z) and errors on malformed input; inst?/inst-ms in
the overlay; pr-str prints the canonical
#inst "yyyy-MM-ddThh:mm:ss.fff-00:00" and round-trips; str gives the bare
RFC3339 string. Self-evaluating in the evaluator (like uuid/chars).

Syntax-quote (jolt-l2a): a syntax-quoted self-evaluating literal (string,
number, boolean, nil, keyword, char) collapses to the literal at READ time,
matching Clojure's reader — so nested/adjacent backticks over literals are
inert: (= "meow" ```"meow") is true (jank pass-adjacent). Symbols still
qualify and collections still template. General nested syntax-quote over
non-literals remains UNVERIFIED in spec S25.

Tests: inst-spec (18 cases incl. partial defaults, offsets, round-trip),
+10 literal-collapse reader rows, +5 conformance rows (321x3). Spec
02-reader S20/S25 updated to normative. Suite stable 4470/86.

test/spec/reader-forms-spec.janet

@@ -63,3 +63,18 @@
   ["var-quote qualified" "true"  "(= (var clojure.core/str) #'clojure.core/str)"]
   ["gensym stable in template" "true" "(let [syms `[meow# meow#]] (= (first syms) (second syms)))"]
   ["gensym fresh across templates" "false" "(= `meow# `meow#)"])
+
+# Spec 02-reader S25: syntax-quote of a self-evaluating literal is the literal
+# (read-time collapse), so adjacent/nested backticks over literals are inert.
+(defspec "reader / syntax-quote literal collapse (spec 2.4 S25)"
+  ["string once"        "true"  "(= \"meow\" `\"meow\")"]
+  ["string nested"      "true"  "(= \"meow\" ``\"meow\")"]
+  ["string triple"      "true"  "(= \"meow\" ```\"meow\")"]
+  ["number nested"      "true"  "(= 42 ``42)"]
+  ["keyword nested"     "true"  "(= :k ``:k)"]
+  ["nil nested"         "true"  "(nil? ``nil)"]
+  ["char nested"        "true"  "(= \\a ``\\a)"]
+  ["bool nested"        "true"  "(= true ``true)"]
+  # collapse must NOT apply to symbols (they qualify) or collections (templates)
+  ["symbol still qualifies" "true" "(= (quote clojure.core/map) `map)"]
+  ["vector still templates" "true" "(= [1 2] `[1 ~(inc 1)])"])