HS: behavior cluster — install + element's subscript fix (+2 tests)

- install BehaviorName: parse-set-cmd handles `element` separately so
  `element's foo` after `set` invokes parse-poss rather than parse-expr,
  fixing `set element's bar["count"] to X` inside behavior bodies
- parse-poss-tail ident case: call parse-poss (handles `[`) instead of
  parse-prop-chain (does not) when next token is bracket-open
- hs-activate!: replace (handler el) with host-call-fn safe wrapper so
  native OCaml "Undefined symbol" throws (which bypass SX guard frames)
  are caught at the JS api_call_fn boundary rather than propagating

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

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

@@ -115,15 +115,21 @@
             (dom-set-data el "hs-script" src)
             (dom-set-data el "hs-active" true)
             (dom-set-attr el "data-hyperscript-powered" "true")
-            (let
-              ((handler (hs-handler src)))
+            (guard
+              (_e (true nil))
               (let
-                ((el-type (dom-get-attr el "type"))
-                  (comp-name (dom-get-attr el "component")))
-                (if
-                  (= el-type "text/hyperscript-template")
-                  (for-each handler (hs-query-all (or comp-name "")))
-                  (handler el))))
+                ((handler (hs-handler src)))
+                (let
+                  ((el-type (dom-get-attr el "type"))
+                    (comp-name (dom-get-attr el "component")))
+                  (let
+                    ((safe-handler (fn (e) (host-call-fn handler (list e)))))
+                    (if
+                      (= el-type "text/hyperscript-template")
+                      (for-each
+                        safe-handler
+                        (hs-query-all (or comp-name "")))
+                      (safe-handler el))))))
             (dom-dispatch el "hyperscript:after:init" nil)))))))
 
 ;; ── Boot subtree: for dynamic content ───────────────────────────