js-on-sx: postfix + prefix ++/--

Parser: jp-parse-postfix emits (js-postfix op target) on trailing
++/--; jp-parse-primary emits (js-prefix op target) before the
unary -/+/!/~ branch.

Transpile: js-transpile-prefix → (set! name (+ (js-to-number name)
±1)) for idents, (js-set-prop obj key ...) for members/indices.
js-transpile-postfix caches old value in a let binding, updates,
returns the saved value.

340/342 unit (+11), 148/148 slice unchanged.

lib/js/test.sh

@@ -875,6 +875,30 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1208)
 (eval "(js-eval \"isFinite(1/0)\")")
 
+;; ── Phase 11.incdec: ++ / -- ─────────────────────────────────────
+(epoch 1300)
+(eval "(js-eval \"var x = 5; x++; x\")")
+(epoch 1301)
+(eval "(js-eval \"var x = 5; var y = x++; y\")")
+(epoch 1302)
+(eval "(js-eval \"var x = 5; ++x\")")
+(epoch 1303)
+(eval "(js-eval \"var x = 5; var y = ++x; y\")")
+(epoch 1304)
+(eval "(js-eval \"var x = 5; x--; x\")")
+(epoch 1305)
+(eval "(js-eval \"var x = 5; --x\")")
+(epoch 1306)
+(eval "(js-eval \"var o = {n: 10}; o.n++; o.n\")")
+(epoch 1307)
+(eval "(js-eval \"var o = {n: 10}; ++o.n\")")
+(epoch 1308)
+(eval "(js-eval \"var a = [1,2,3]; a[1]++; a[1]\")")
+(epoch 1309)
+(eval "(js-eval \"var x = 5; for (var i = 0; i < 3; i++) x++; x\")")
+(epoch 1310)
+(eval "(js-eval \"var sum = 0; for (var i = 1; i <= 5; i++) sum = sum + i; sum\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1340,6 +1364,19 @@ check 1206 "Number.EPSILON > 0"            'true'
 check 1207 "isFinite(1)"                   'true'
 check 1208 "isFinite(Inf)"                 'false'
 
+# ── Phase 11.incdec: ++ / -- ────────────────────────────────────
+check 1300 "x++ bumps x"                    '6'
+check 1301 "x++ returns old"                '5'
+check 1302 "++x bumps and returns"          '6'
+check 1303 "++x captured new"               '6'
+check 1304 "x-- decrements"                 '4'
+check 1305 "--x decrements, returns new"    '4'
+check 1306 "obj.n++"                        '11'
+check 1307 "++obj.n"                        '11'
+check 1308 "a[1]++"                         '3'
+check 1309 "for-loop x++"                   '8'
+check 1310 "for-loop accumulator"           '15'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"