core: edn :readers/:default opts; uncaught throws no longer leak the callee's ns

clojure.edn was nearly complete (sets, #uuid/#inst, :eof all landed earlier);
the :readers opt was ignored and :default missing. Both work now — the
reader stores a tag as a :#name keyword, so the lookup normalizes it to the
symbol Clojure keys :readers with; :default gets (tag value); built-in data
readers stay the fallback. 8 new edn spec rows. This was the last open item
of jolt-0mb (the vendored walk/zip/data/edn battery has been green for a
while: 34/33/61/50, all clean).

Chasing the probe cascade ('Unable to resolve symbol: edn/...' after one
error) found a real evaluator bug: an interpreted fn body runs with
current-ns rebound to its DEFINING ns and restores it with a plain trailing
call — an UNCAUGHT throw skips every restore on the way out, leaving the ctx
stuck in the deepest callee's ns, where alias-qualified lookups then fail
(the same cascade previously seen via sci). The repair lives at the
TOP-LEVEL boundary (loader/eval-toplevel saves the entry ns and restores it
on error before re-raising) — NOT per-call defer/try, which builds a fiber
per frame and blew the C stack on deep interpreted recursion (file-seq)
when tried first. Regression tests cover the cross-eval leak and that
aliases keep resolving.

test/integration/namespace-test.janet

@@ -96,3 +96,20 @@
 (print "  passed")
 
 (print "\nAll namespace tests passed!")
+
+# An UNCAUGHT throw inside an interpreted fn body must restore the caller's
+# current-ns (the body runs with current-ns rebound to the defining ns; the
+# restore is a defer now). Pre-fix, the ctx was left stuck in the defining ns
+# and every later alias-qualified lookup failed — the sci-bootstrap and
+# clojure.edn "Unable to resolve alias/..." cascades.
+(print "ns restored after uncaught throw...")
+(let [ctx (fresh-ctx)]
+  (api/eval-string ctx "(in-ns (quote otherns))")
+  (api/eval-string ctx "(clojure.core/refer-clojure)")
+  (api/eval-string ctx "(defn boom [] (throw (ex-info \"x\" {})))")
+  (api/eval-string ctx "(in-ns (quote user))")
+  (assert (not ((protect (api/eval-string ctx "(otherns/boom)")) 0)) "boom throws")
+  (assert (= "user" (api/eval-string ctx "(str *ns*)")) "*ns* restored after uncaught throw")
+  (api/eval-string ctx "(require (quote [clojure.string :as s9]))")
+  (assert (= "A" (api/eval-string ctx "(s9/upper-case \"a\")")) "aliases still resolve"))
+(print "  ok")

test/spec/namespaces-spec.janet

@@ -42,3 +42,14 @@
    "(do (require (quote clojure.string)) (alias (quote st2) (quote clojure.string)) (ns-unalias (quote user) (quote st2)) (nil? (get (ns-aliases (quote user)) (quote st2))))"]
   ["ns-publics has var" "true"  "(do (def npv 1) (some? (get (ns-publics (quote user)) (quote npv))))"]
   ["newline returns nil" "nil"  "(newline)"])
+
+# A throw inside an interpreted fn body (or macro expander) must restore the
+# caller's current-ns: the body runs with current-ns rebound to the fn's
+# DEFINING ns, and an unwind that skipped the restore left the ctx stuck
+# there — every later alias-qualified lookup in the REPL ns then failed
+# ("Unable to resolve symbol: alias/...", seen via sci + clojure.edn).
+(defspec "namespaces / error inside a fn must not leak its defining ns"
+  ["alias survives a throwing stdlib call" "\"A\""
+   "(do (require (quote [clojure.string :as s9])) (try (s9/join nil nil nil) (catch Exception e nil)) (s9/upper-case \"a\"))"]
+  ["*ns* restored after throw" "\"user\""
+   "(do (require (quote [clojure.walk :as w9])) (try (w9/postwalk nil nil nil) (catch Exception e nil)) (str *ns*))"])

test/spec/stage3-io-ns-spec.janet

@@ -37,3 +37,16 @@
   ["load-string evals all" "3"  "(load-string \"(def lsv 1) (+ lsv 2)\")"]
   ["eval as value"       "[2 3]" "(mapv eval [(quote (+ 1 1)) (quote (+ 1 2))])"]
   ["eval special still works" "3" "(eval (quote (+ 1 2)))"])
+
+# clojure.edn is complete (jolt-b7y / jolt-0mb): sets, #uuid/#inst, :eof,
+# and the :readers / :default opts (tag normalized from the reader's :#name
+# keyword to the symbol Clojure keys :readers with).
+(defspec "clojure.edn / opts"
+  ["set literal"     "#{1 2}" "(do (require (quote [clojure.edn :as e0])) (e0/read-string \"#{1 2}\"))"]
+  ["uuid tag"        "true"   "(do (require (quote [clojure.edn :as e0])) (uuid? (e0/read-string \"#uuid \\\"550e8400-e29b-41d4-a716-446655440000\\\"\")))"]
+  ["inst tag"        "true"   "(do (require (quote [clojure.edn :as e0])) (inst? (e0/read-string \"#inst \\\"2020-01-01T00:00:00Z\\\"\")))"]
+  [":eof on empty"   ":end"   "(do (require (quote [clojure.edn :as e0])) (e0/read-string {:eof :end} \"\"))"]
+  [":readers custom tag" "[:custom 5]" "(do (require (quote [clojure.edn :as e0])) (e0/read-string {:readers {(quote custom) (fn [v] [:custom v])}} \"#custom 5\"))"]
+  [":readers nested" "[6 8]"  "(do (require (quote [clojure.edn :as e0])) (e0/read-string {:readers {(quote w) (fn [v] (* 2 v))}} \"[#w 3 #w 4]\"))"]
+  [":default fn"     "[:dflt 7]" "(do (require (quote [clojure.edn :as e0])) (e0/read-string {:default (fn [t v] [:dflt v])} \"#unknown 7\"))"]
+  ["unknown tag throws" :throws "(do (require (quote [clojure.edn :as e0])) (e0/read-string \"#nope 1\"))"])