js-on-sx: optional chaining ?.

Parser: jp-parse-postfix handles op "?." followed by ident / [ / (
emitting (js-optchain-member obj name), (js-optchain-index obj k),
or (js-optchain-call callee args).

Transpile: each emits (js-optchain-get obj key) or (js-optchain-call
fn args).

Runtime: js-optchain-get and js-optchain-call short-circuit to
js-undefined when receiver is null/undefined.

423/425 unit (+5), 148/148 slice unchanged.

lib/js/test.sh

@@ -1081,6 +1081,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2603)
 (eval "(js-eval \"var pt = {px: 100}; var {px} = pt; px + 1\")")
 
+;; ── Phase 11.optchain: ?. optional chaining ─────────────────────
+(epoch 2700)
+(eval "(js-eval \"var o = {x: 5}; o?.x\")")
+(epoch 2701)
+(eval "(js-eval \"var o = null; o?.x\")")
+(epoch 2702)
+(eval "(js-eval \"var o = undefined; o?.x\")")
+(epoch 2703)
+(eval "(js-eval \"var o = {a:{b:7}}; o?.a?.b\")")
+(epoch 2704)
+(eval "(js-eval \"var o = {a:null}; var r = o?.a?.b; r === undefined\")")
+
 EPOCHS
 
 
@@ -1664,6 +1676,13 @@ check 2601 "arr destructure"               '6'
 check 2602 "arr destructure skip"          '4'
 check 2603 "obj partial+add"               '101'
 
+# ── Phase 11.optchain ──────────────────────────────────────────
+check 2700 "?. obj present"                '5'
+check 2701 "?. obj null"                   'undefined'
+check 2702 "?. obj undef"                  'undefined'
+check 2703 "?. chained"                    '7'
+check 2704 "?. null-chain"                 'true'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"