HS: transition to initial + commit pending E37/E40 test impls

parser.sx: detect bare ident "initial" after "to" in parse-one-transition,
  emit string sentinel instead of (ref "initial") which evaluated to nil.
runtime.sx: hs-transition stores pre-first-transition style as
  data-hs-init-{prop}; restores it when value=="initial".

Also commits E37 tokenizer and E40 fetch test implementations that
accumulated in the working tree but weren't staged in prior commits.

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

lib/hyperscript/runtime.sx

@@ -1362,14 +1362,21 @@
   hs-transition
   (fn
     (target prop value duration)
-    (when
-      duration
-      (dom-set-style
-        target
-        "transition"
-        (str prop " " (/ duration 1000) "s")))
-    (dom-set-style target prop value)
-    (when duration (hs-settle target))))
+    (let
+      ((init-attr (str "data-hs-init-" prop)))
+      (when
+        (not (dom-get-attr target init-attr))
+        (dom-set-attr target init-attr (dom-get-style target prop)))
+      (let
+        ((actual-value (if (= value "initial") (dom-get-attr target init-attr) value)))
+        (when
+          duration
+          (dom-set-style
+            target
+            "transition"
+            (str prop " " (/ duration 1000) "s")))
+        (dom-set-style target prop actual-value)
+        (when duration (hs-settle target))))))
 
 (define
   hs-transition-from