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>

tests/playwright/generate-sx-tests.py

@@ -326,6 +326,27 @@ MANUAL_TEST_BODIES = {
         '          (list (list (quote record) record)))',
         '        "<div age=\\"21\\" style=\\"color:bleaux\\">John Connor</div>"))',
     ],
+    # asExpression: custom conversions — set/clear via hs-set-conversion! + hs-add-dynamic-converter!
+    "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"))))',
+    ],
+    "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"))))',
+    ],
     # asExpression: Date/Set/Map need real JS host objects
     "converts value as Date": [
         '    (let ((_result (eval-hs "1 as Date")))',