core: java shims for yogthos/config + three conformance fixes

yogthos/config now loads and runs end to end: config.edn/.lein-env
deep merge, env vars keywordized and type-converted, PushbackReader
over io/reader, reload-env via alter-var-root. New shims:
java.io.PushbackReader (read/unread), Long/parseLong, BigInteger.,
Boolean/parseBoolean, System/getProperties; clojure.edn/read now
drains an actual reader (it used to read one LINE from a raw janet
file handle, so multi-line config files and shim readers both broke).

Three real bugs shaken out, each with regression specs:

- An empty rest arg bound () instead of nil. ((fn [& r] (if r 1 2)))
  returned 1; the truthy () sent config's (or (.exists f) required)
  down the wrong branch. Fixed in the interpreter and the compiled-fn
  emission. Internal apply boundaries (protocol dispatch, core-apply)
  now accept a nil seq like Clojure's (apply f x nil).

- seq/map over a raw janet table (System/getenv, os/environ) yielded
  nothing, so config's read-system-env came back empty. Raw host
  tables now seq as kv entries like any map, in core-seq,
  realize-for-iteration, and coll->cells. The old spec row hid this
  behind a vacuous (every? pred empty) — replaced with one that
  asserts non-emptiness.

- edn/read single-line limitation, as above.

test/integration/config-lib-test.janet runs the real library from
~/src/config (skips when absent).

test/spec/functions-spec.janet

@@ -181,3 +181,16 @@
    "((fn f [acc & xs] (if (seq xs) (recur acc (rest xs)) :empty)) 0 1)"]
   ["fixed-arity recur untouched" "10"
    "((fn f [n acc] (if (pos? n) (recur (dec n) (+ acc 2)) acc)) 5 0)"])
+
+# An EMPTY rest arg binds nil, never () — Clojure semantics. (f) with [& r]
+# giving a truthy () sent yogthos/config's (or (.exists f) required) down the
+# wrong branch and broke its load.
+(defspec "functions / empty rest arg is nil"
+  ["no args"            ":nil"  "((fn [& r] (if r :truthy :nil)))"]
+  ["after fixed"        ":nil"  "((fn [a & r] (if r :truthy :nil)) 1)"]
+  ["via apply"          ":nil"  "(apply (fn [& r] (if r :truthy :nil)) [])"]
+  ["defn no args"       ":nil"  "(do (defn er-f [& r] (if r :truthy :nil)) (er-f))"]
+  ["non-empty unchanged" "'(1 2)" "((fn [& r] r) 1 2)"]
+  ["one extra"          "'(2)"   "((fn [a & r] r) 1 2)"]
+  ["rest destructure with no args" ":nil"
+   "((fn [& [a]] (if a :truthy :nil)))"])