test: restructure into unit / integration / spec layers + shared harness

Reorganize the flat 49-file test/ into three layers (jpm test recurses, so all
are still discovered):
- test/unit/        white-box component tests (reader, evaluator, types,
                    persistent-map, lazy-seq, macro, interop, compiler)
- test/integration/ cross-cutting + regression batteries (conformance, jank,
                    sci-bootstrap/runtime, features, systematic-coverage, api,
                    core, namespaces, ported clojure suites) and
   .../ports/       ported clojure/cljs test batches pending consolidation
- test/spec/        the behavioral contract (built out in following commits)
- test/support/harness.janet  shared defspec table runner (cases compared via
                    Jolt's own =, with a :throws sentinel) + expect= helpers

Files moved with git mv (history preserved) and import paths fixed for depth.
jpm test green. README Test section updated.

Next: build out test/spec/ to cover the public API area-by-area, mining the
integration batteries and filling gaps.

test/unit/interop-test.janet

@@ -0,0 +1,46 @@
+(use ../../src/jolt/api)
+(defn ct-eval [ctx s] (normalize-pvecs (eval-string ctx s)))
+(print "Janet Interop Tests")
+
+(print "1: field access on tables...")
+(let [ctx (init)]
+  (ct-eval ctx "(def t {:a 1 :b 2})")
+  (assert (= 1 (ct-eval ctx "(. t :a)")) ". field access table")
+  (assert (= 2 (ct-eval ctx "(. t :b)")) ". field access table 2")
+  (assert (= nil (ct-eval ctx "(. t :z)")) ". field access missing key"))
+(print "  ok")
+
+(print "2: field access on structs...")
+(let [ctx (init)]
+  (ct-eval ctx "(def s {:x 10 :y 20})")
+  (assert (= 10 (ct-eval ctx "(. s :x)")) ". field access struct")
+  (assert (= 20 (ct-eval ctx "(. s :y)")) ". field access struct 2"))
+(print "  ok")
+
+(print "3: method calls on tables...")
+(let [ctx (init)]
+  (ct-eval ctx "(def obj {:greet (fn [self name] (str \"Hello \" name))})")
+  (assert (= "Hello Alice" (ct-eval ctx "(. obj greet \"Alice\")")) ". method call on table"))
+(print "  ok")
+
+(print "4: field access via .- reader sugar...")
+(let [ctx (init)]
+  (ct-eval ctx "(def t {:x 42 :len 5})")
+  (assert (= 42 (ct-eval ctx "(.-x t)")) ".-x reader sugar")
+  (assert (= 5 (ct-eval ctx "(.-len t)")) ".-len reader sugar"))
+(print "  ok")
+
+(print "5: . field access still works on deftypes...")
+(let [ctx (init)]
+  (ct-eval ctx "(deftype Point [x y])")
+  (assert (= 3 (ct-eval ctx "(. (Point. 3 4) x)")) ". field access deftype"))
+(print "  ok")
+
+(print "6: method call with multiple args...")
+(let [ctx (init)]
+  (ct-eval ctx "(def calc {:add (fn [_ a b] (+ a b)) :mul (fn [_ a b] (* a b))})")
+  (assert (= 7 (ct-eval ctx "(. calc add 3 4)")) ". method add")
+  (assert (= 12 (ct-eval ctx "(. calc mul 3 4)")) ". method mul"))
+(print "  ok")
+
+(print "\nAll Janet Interop tests passed!")