HS: parser fixes — parenthesized commands + add error + class-name depth (+3 tests)

- parse-on-feat: event-vars paren check now restores position and returns empty
  list when the first token after '(' is a keyword (command starter). Previously
  '(log me)' was consumed as event variable names instead of a parenthesized
  command, silently dropping the command body and returning empty innerHTML.
  Fixes 'can support parenthesized commands and features'.

- parse-add-cmd: true-fallback now throws instead of returning nil when no 'to'
  keyword follows the expression. Makes 'add - to' and similar invalid add forms
  throw a parse error, satisfying assert-throws in 'basic parse error messages
  work' and '_hyperscript() evaluate API still throws on first error'.

- read-class-name: '(' and ')' now only allowed inside '[...]' bracket groups
  (depth > 0). Previously allowing them at top level caused '.innerHTML)' at the
  end of a possessive expression to be consumed into the class token, producing
  'innerHTML))' as a bogus property name. Tailwind classes like
  'group-[:nth-of-type(3)_&]:block' still tokenize correctly.

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

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

@@ -459,7 +459,7 @@
           (define
             build-name
             (fn
-              (acc)
+              (acc depth)
               (cond
                 ((and (< pos src-len) (= (hs-cur) "\\") (< (+ pos 1) src-len))
                   (do
@@ -467,15 +467,35 @@
                     (let
                       ((c (hs-cur)))
                       (hs-advance! 1)
-                      (build-name (str acc c)))))
-                ((and (< pos src-len) (or (hs-ident-char? (hs-cur)) (= (hs-cur) ":") (= (hs-cur) "[") (= (hs-cur) "]") (= (hs-cur) "(") (= (hs-cur) ")") (= (hs-cur) "&")))
+                      (build-name (str acc c) depth))))
+                ((and (< pos src-len) (= (hs-cur) "["))
                   (do
                     (let
                       ((c (hs-cur)))
                       (hs-advance! 1)
-                      (build-name (str acc c)))))
+                      (build-name (str acc c) (+ depth 1)))))
+                ((and (< pos src-len) (= (hs-cur) "]"))
+                  (do
+                    (let
+                      ((c (hs-cur)))
+                      (hs-advance! 1)
+                      (build-name
+                        (str acc c)
+                        (if (> depth 0) (- depth 1) 0)))))
+                ((and (< pos src-len) (> depth 0) (or (= (hs-cur) "(") (= (hs-cur) ")")))
+                  (do
+                    (let
+                      ((c (hs-cur)))
+                      (hs-advance! 1)
+                      (build-name (str acc c) depth))))
+                ((and (< pos src-len) (or (hs-ident-char? (hs-cur)) (= (hs-cur) ":") (= (hs-cur) "&")))
+                  (do
+                    (let
+                      ((c (hs-cur)))
+                      (hs-advance! 1)
+                      (build-name (str acc c) depth))))
                 (true acc))))
-          (build-name "")))
+          (build-name "" 0)))
       (define
         hs-emit!
         (fn