HS: custom conversion API + asExpression tests (+2)

Add _hs-custom-conversions dict and _hs-dynamic-converters list to
runtime.sx. hs-set-conversion!/hs-clear-conversion!/hs-add-dynamic-converter!/
hs-pop-dynamic-converter!/hs-clear-converters! helpers expose the API.
hs-coerce fallback now checks static dict then dynamic resolvers before
returning value unchanged.

Hand-roll MANUAL_TEST_BODIES for "can accept custom conversions" and
"can accept custom dynamic conversions" — previously SKIP (untranslated).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/tests/test-hyperscript-behavioral.sx

@@ -3724,9 +3724,25 @@
 ;; ── expressions/asExpression (42 tests) ──
 (defsuite "hs-upstream-expressions/asExpression"
   (deftest "can accept custom conversions"
-    (error "SKIP (untranslated): can accept custom conversions"))
+    (do
+      (hs-set-conversion! "Foo" (fn (val) (str "foo" (str val))))
+      (let ((result (hs-coerce 1 "Foo")))
+        (do
+          (hs-clear-conversion! "Foo")
+          (assert= result "foo1"))))
+  )
   (deftest "can accept custom dynamic conversions"
-    (error "SKIP (untranslated): can accept custom dynamic conversions"))
+    (do
+      (hs-add-dynamic-converter!
+        (fn (conversion val)
+          (if (= (host-call conversion "indexOf" "Foo:") 0)
+            (str (host-call conversion "slice" 4) (str val))
+            nil)))
+      (let ((result (hs-coerce 1 "Foo:Bar")))
+        (do
+          (hs-pop-dynamic-converter!)
+          (assert= result "Bar1"))))
+  )
   (deftest "can use the a modifier if you like"
     (let ((_result (eval-hs "1 as a Date")))
       (assert= (host-call _result "getTime") 1))