js-on-sx: Array.prototype flat + fill; fix indexOf start arg

flat: walk with depth, recursive when element is list and depth>0.
fill(value, start?, end?): in-place mutation, returns self.
indexOf: honor second arg as start position.

396/398 unit (+5), 148/148 slice unchanged.

lib/js/test.sh

@@ -1015,6 +1015,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2009)
 (eval "(js-eval \"JSON.parse('{\\\"a\\\":1}').a\")")
 
+;; ── Phase 11.array2: flat + fill + indexOf start ───────────────
+(epoch 2100)
+(eval "(js-eval \"[1,[2,3],4].flat().length\")")
+(epoch 2101)
+(eval "(js-eval \"[1,[2,[3]]].flat(2).length\")")
+(epoch 2102)
+(eval "(js-eval \"[1,2,3].fill(0).join(',')\")")
+(epoch 2103)
+(eval "(js-eval \"[1,2,3,4].fill(0, 1, 3).join(',')\")")
+(epoch 2104)
+(eval "(js-eval \"[1,2,1,2].indexOf(2, 2)\")")
+
 EPOCHS
 
 
@@ -1559,6 +1571,13 @@ check 2007 "parse string"                   '"hello"'
 check 2008 "parse array length"             '3'
 check 2009 "parse object.a"                 '1'
 
+# ── Phase 11.array2 ────────────────────────────────────────────
+check 2100 "flat() depth 1"                '4'
+check 2101 "flat(2)"                       '3'
+check 2102 "fill(0)"                       '"0,0,0"'
+check 2103 "fill(0,1,3)"                   '"1,0,0,4"'
+check 2104 "indexOf with start"            '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"