HS: parse-cmd arith guard fixes — math/numbers/sourceInfo/stringPostfix (+14 tests)

Three parse-cmd / parse-feat refinements:

1. Remove dict-branch from arith guard: span-mode=true produces dict nodes
   with :kind "arith", not lists. The guard only needs the list-branch (for
   span-mode=false). Without this, hs-src "x + y" threw a parse error.

2. parse-feat top-level expression-first fallback: when no feature keyword is
   found, try parse-expr first. If it fully consumes the input (at-end?),
   return the expression directly — bypassing parse-cmd and its arith guard.
   This matches upstream _hyperscript("1 + 1") which evaluates as an
   expression, not a pseudo-command.

3. paren-close exception in arith guard: when the token after the arithmetic
   expression is ")", we are inside a parenthesised context (e.g. "(0+1) em"
   string-postfix). Allow it through without the pseudo-command error.

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

shared/static/wasm/sx/hs-parser.sx

@@ -2878,19 +2878,17 @@
                   (let
                     ((expr (parse-expr)))
                     (if
-                      (if
-                        (and (dict? expr) (get expr :hs-ast))
-                        (= (get expr :kind) "arith")
-                        (and
-                          (list? expr)
-                          (let
-                            ((h (first expr)))
-                            (or
-                              (= h (quote +))
-                              (= h (quote -))
-                              (= h (quote *))
-                              (= h (quote /))
-                              (= h (make-symbol "%"))))))
+                      (and
+                        (list? expr)
+                        (not (= (tp-type) "paren-close"))
+                        (let
+                          ((h (first expr)))
+                          (or
+                            (= h (quote +))
+                            (= h (quote -))
+                            (= h (quote *))
+                            (= h (quote /))
+                            (= h (make-symbol "%")))))
                       (error "Pseudo-commands must be function calls")
                       expr))))))))
       (define
@@ -3220,7 +3218,15 @@
                 (error
                   "worker plugin is not installed — see https://hyperscript.org/features/worker"))
               ((= val "bind") (do (adv!) (parse-bind-feat)))
-              (true (parse-cmd-list))))))
+              (true
+                (let
+                  ((saved-p p))
+                  (let
+                    ((expr (guard (_e (true nil)) (parse-expr))))
+                    (if
+                      (and expr (at-end?))
+                      expr
+                      (do (set! p saved-p) (parse-cmd-list))))))))))
       (define
         coll-feats
         (fn