Phase 6: Reader Extensions — tagged literals (#inst, #uuid)

- evaluator.janet: add jolt/tagged form handler in eval-form
  resolves tag via :data-readers from context env
- types.janet: :data-readers in make-ctx env with #inst/#uuid
  passthrough readers (return strings on Janet, no java.time/UUID)
- Dynamic table construction for data-readers keys
  (:#inst keyword containing # is invalid Janet literal syntax)
- test/phase6-test.janet: 4 test sections (28-31)
  #inst, #uuid, #? conditionals, #?@ splicing — all pass
- 315 ok, 2 fail (pre-existing, unchanged)

test/phase6-test.janet

@@ -0,0 +1,33 @@
+# Phase 6: Reader Extensions Tests
+(use ../src/jolt/api)
+(defn ct-eval [ctx s] (eval-string ctx s))
+
+(print "28: #inst tagged literal...")
+(let [ctx (init)]
+  (let [val (ct-eval ctx "#inst \"2024-01-15\"")]
+    (assert (= "2024-01-15" val) "#inst string"))
+  (let [val (ct-eval ctx "#inst \"2024-01-15T10:30:00Z\"")]
+    (assert (= "2024-01-15T10:30:00Z" val) "#inst timestamp")))
+(print "  passed")
+
+(print "29: #uuid tagged literal...")
+(let [ctx (init)]
+  (let [val (ct-eval ctx "#uuid \"550e8400-e29b-41d4-a716-446655440000\"")]
+    (assert (= "550e8400-e29b-41d4-a716-446655440000" val) "#uuid string")))
+(print "  passed")
+
+(print "30: #? reader conditionals...")
+(let [ctx (init)]
+  (assert (= :yes (ct-eval ctx "#?(:clj :yes :cljs :no)")) "#? selects :clj")
+  (assert (= nil (ct-eval ctx "#?(:cljs :no)")) "#? nil on no match"))
+(print "  passed")
+
+(print "31: #?@ splicing...")
+(let [ctx (init)]
+  (assert (= [1 2 3] (ct-eval ctx "[#?@(:clj [1 2 3] :cljs [4 5 6])]"))
+          "#?@ splices :clj")
+  (assert (= [] (ct-eval ctx "[#?@(:cljs [1 2])]"))
+          "#?@ nothing on no match"))
+(print "  passed")
+
+(print "\nAll Phase 6 tests passed!")