js-on-sx: logical assignment &&= ||= ??=

js-compound-update gains logical-assign operators:
- &&= → (if (js-to-boolean lhs) rhs lhs)
- ||= → (if (js-to-boolean lhs) lhs rhs)
- ??= → (if nullish? rhs lhs)

427/429 unit (+4), 148/148 slice unchanged.

lib/js/test.sh

@@ -1093,6 +1093,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2704)
 (eval "(js-eval \"var o = {a:null}; var r = o?.a?.b; r === undefined\")")
 
+;; ── Phase 11.logassign: &&= ||= ??= ─────────────────────────────
+(epoch 2800)
+(eval "(js-eval \"var q=0; q||=5; q\")")
+(epoch 2801)
+(eval "(js-eval \"var q=1; q&&=7; q\")")
+(epoch 2802)
+(eval "(js-eval \"var q=null; q??=99; q\")")
+(epoch 2803)
+(eval "(js-eval \"var q=42; q??=99; q\")")
+
 EPOCHS
 
 
@@ -1683,6 +1693,12 @@ check 2702 "?. obj undef"                  'undefined'
 check 2703 "?. chained"                    '7'
 check 2704 "?. null-chain"                 'true'
 
+# ── Phase 11.logassign ────────────────────────────────────────
+check 2800 "||= falsy"                      '5'
+check 2801 "&&= truthy"                     '7'
+check 2802 "??= null"                       '99'
+check 2803 "??= has value"                  '42'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"