js-on-sx: delete operator

js-transpile-unop intercepts 'delete' before transpiling the
operand. Maps to (js-delete-prop obj key) for members and indexed
access. Runtime js-delete-prop sets the dict value to js-undefined
and returns true.

444/446 unit (+2), 148/148 slice unchanged.

lib/js/test.sh

@@ -1141,6 +1141,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3201)
 (eval "(js-eval \"var {aa: x2, bb: y2} = {aa:1, bb:2}; x2+y2\")")
 
+;; ── Phase 11.delete ────────────────────────────────────────────
+(epoch 3300)
+(eval "(js-eval \"var o = {x: 5}; delete o.x; o.x === undefined\")")
+(epoch 3301)
+(eval "(js-eval \"var o = {a:1, b:2}; delete o.a; o.b\")")
+
 EPOCHS
 
 
@@ -1760,6 +1766,10 @@ check 3101 "rest arr join"                 '"2,3,4"'
 check 3200 "obj rename"                    '100'
 check 3201 "obj multi rename"              '3'
 
+# ── Phase 11.delete ───────────────────────────────────────────
+check 3300 "delete obj.x"                  'true'
+check 3301 "delete obj.a keeps b"         '2'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"