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>

lib/hyperscript/runtime.sx

@@ -1156,6 +1156,41 @@
             "}")))
       (true (hs-json-escape (str v))))))
 
+(begin
+  (define _hs-custom-conversions {})
+  (define _hs-dynamic-converters (list))
+  (define
+    hs-set-conversion!
+    (fn (name conv-fn) (dict-set! _hs-custom-conversions name conv-fn)))
+  (define
+    hs-clear-conversion!
+    (fn (name) (dict-set! _hs-custom-conversions name nil)))
+  (define
+    hs-add-dynamic-converter!
+    (fn
+      (conv-fn)
+      (set!
+        _hs-dynamic-converters
+        (append _hs-dynamic-converters (list conv-fn)))))
+  (define
+    hs-pop-dynamic-converter!
+    (fn
+      ()
+      (let
+        ((n (len _hs-dynamic-converters)))
+        (when
+          (> n 0)
+          (set!
+            _hs-dynamic-converters
+            (slice _hs-dynamic-converters 0 (- n 1)))))))
+  (define
+    hs-clear-converters!
+    (fn
+      ()
+      (do
+        (set! _hs-custom-conversions {})
+        (set! _hs-dynamic-converters (list))))))
+
 (define
   hs-coerce
   (fn
@@ -1280,7 +1315,15 @@
                 (filter (fn (k) (not (= k "_order"))) (keys value))))
             m)))
       ((= type-name "Date") (host-new "Date" value))
-      (true value))))
+      (true
+        (let
+          ((static-fn (get _hs-custom-conversions type-name)))
+          (if
+            (not (nil? static-fn))
+            (static-fn value)
+            (let
+              ((dynamic-result (reduce (fn (acc resolver) (if (not (nil? acc)) acc (resolver type-name value))) nil _hs-dynamic-converters)))
+              (if (not (nil? dynamic-result)) dynamic-result value))))))))
 
 (define
   hs-gather-form-nodes
@@ -2722,6 +2765,8 @@
       ((store (host-get el "__hs_vars")))
       (if (nil? store) nil (host-get store name)))))
 
+;; ── SourceInfo API ────────────────────────────────────────────────
+
 (define
   hs-dom-set-var-raw!
   (fn
@@ -2735,8 +2780,6 @@
         (host-set! (host-get el "__hs_vars") name val)
         (when changed (hs-dom-fire-watchers! el name val))))))
 
-;; ── SourceInfo API ────────────────────────────────────────────────
-
 (define
   hs-dom-resolve-start
   (fn