HS empty + .length/.size on SX collections

- compiler emit-empty-target: for empty :local, rebind via emit-set so
  scoped locals persist.
- tests/hs-run-filtered.js host-get: when reading length/size from an SX
  list, return items.length. Similarly for SX dict size (non-_type keys).
  Unlocks :arr.length / :set.size / :map.size in compiled HS.

Net: empty 8→11, remove 17→18, add (set dedup) exposed as separate issue.

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

@@ -1257,13 +1257,12 @@
               ((= head (quote empty-target))
                 (let
                   ((tgt (nth ast 1)))
-                  (if
-                    (and (list? tgt) (= (first tgt) (quote local)))
-                    (list
-                      (quote set!)
-                      (make-symbol (nth tgt 1))
-                      (list (quote hs-empty-like) (make-symbol (nth tgt 1))))
-                    (list (quote hs-empty-target!) (hs-to-sx tgt)))))
+                  (cond
+                    ((and (list? tgt) (= (first tgt) (quote local)))
+                      (emit-set
+                        tgt
+                        (list (quote hs-empty-like) (hs-to-sx tgt))))
+                    (true (list (quote hs-empty-target!) (hs-to-sx tgt))))))
               ((= head (quote open-element))
                 (list (quote hs-open!) (hs-to-sx (nth ast 1))))
               ((= head (quote close-element))