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>

lib/hyperscript/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 ───────────────────────────

lib/hyperscript/parser.sx

@@ -73,7 +73,12 @@
               ((or (= typ "ident") (= typ "keyword"))
                 (do
                   (adv!)
-                  (parse-prop-chain (list (quote poss) owner val))))
+                  (let
+                    ((base (list (quote poss) owner val)))
+                    (if
+                      (= (tp-type) "bracket-open")
+                      (parse-poss base)
+                      (parse-prop-chain base)))))
               ((= typ "attr") (do (adv!) (list (quote attr) val owner)))
               ((= typ "class")
                 (let
@@ -1555,7 +1560,7 @@
         (fn
           ()
           (let
-            ((tgt-raw (cond ((and (= (tp-type) "ident") (or (= (tp-val) "element") (= (tp-val) "global") (= (tp-val) "local"))) (do (adv!) (parse-expr))) (true (parse-expr)))))
+            ((tgt-raw (cond ((and (= (tp-type) "ident") (or (= (tp-val) "global") (= (tp-val) "local"))) (do (adv!) (parse-expr))) ((and (= (tp-type) "ident") (= (tp-val) "element")) (do (adv!) (if (and (= (tp-type) "op") (= (tp-val) "'s")) (parse-poss (list (quote ref) "element")) (parse-expr)))) (true (parse-expr)))))
             (let
               ((tgt (if (= (tp-type) "attr") (let ((attr-name (get (adv!) "value"))) (list (quote attr) attr-name tgt-raw)) tgt-raw)))
               (cond

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 ───────────────────────────

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

@@ -73,7 +73,12 @@
               ((or (= typ "ident") (= typ "keyword"))
                 (do
                   (adv!)
-                  (parse-prop-chain (list (quote poss) owner val))))
+                  (let
+                    ((base (list (quote poss) owner val)))
+                    (if
+                      (= (tp-type) "bracket-open")
+                      (parse-poss base)
+                      (parse-prop-chain base)))))
               ((= typ "attr") (do (adv!) (list (quote attr) val owner)))
               ((= typ "class")
                 (let
@@ -1555,7 +1560,7 @@
         (fn
           ()
           (let
-            ((tgt-raw (cond ((and (= (tp-type) "ident") (or (= (tp-val) "element") (= (tp-val) "global") (= (tp-val) "local"))) (do (adv!) (parse-expr))) (true (parse-expr)))))
+            ((tgt-raw (cond ((and (= (tp-type) "ident") (or (= (tp-val) "global") (= (tp-val) "local"))) (do (adv!) (parse-expr))) ((and (= (tp-type) "ident") (= (tp-val) "element")) (do (adv!) (if (and (= (tp-type) "op") (= (tp-val) "'s")) (parse-poss (list (quote ref) "element")) (parse-expr)))) (true (parse-expr)))))
             (let
               ((tgt (if (= (tp-type) "attr") (let ((attr-name (get (adv!) "value"))) (list (quote attr) attr-name tgt-raw)) tgt-raw)))
               (cond