js-on-sx: 15 new Array.prototype methods (at, flatMap, findLast, reduceRight, toString, toReversed, toSorted, ...)

New read-only methods added:
- at(i) — negative-index aware
- flatMap(f) — map then flatten one level
- findLast(f) / findLastIndex(f)
- reduceRight(f, init?)
- toString / toLocaleString — join with ','
- keys() / values() / entries() — index/value/pair lists
- copyWithin(target, start, end) — in-place via set-nth!
- toReversed() / toSorted() — non-mutating variants

Mutating methods (unshift, splice) are stubs that return correct lengths
but don't mutate — we don't have a pop-first!/clear! primitive to rebuild
the list in place. Tracked as a runtime limitation.

10 new unit tests, 479/481 total. Directly targets the 785x
ReferenceError in built-ins/Array and the many .toString() crashes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/js/test.sh

@@ -1191,6 +1191,28 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3609)
 (eval "(js-eval \"(true).valueOf()\")")
 
+;; ── Phase 11.arrmore: more Array.prototype methods ──────────
+(epoch 3700)
+(eval "(js-eval \"[1,2,3].at(0)\")")
+(epoch 3701)
+(eval "(js-eval \"[1,2,3].at(-1)\")")
+(epoch 3702)
+(eval "(js-eval \"[1,2,3,4].flatMap(x=>[x,x*2]).join(',')\")")
+(epoch 3703)
+(eval "(js-eval \"[1,5,2,7,3].findLast(x=>x<5)\")")
+(epoch 3704)
+(eval "(js-eval \"[1,5,2,7,3].findLastIndex(x=>x<5)\")")
+(epoch 3705)
+(eval "(js-eval \"[1,2,3,4].reduceRight((acc,x)=>acc+','+x)\")")
+(epoch 3706)
+(eval "(js-eval \"[1,2,3].toString()\")")
+(epoch 3707)
+(eval "(js-eval \"[3,1,2].toReversed().join(',')\")")
+(epoch 3708)
+(eval "(js-eval \"[3,1,4,1,5].toSorted((a,b)=>a-b).join(',')\")")
+(epoch 3709)
+(eval "(js-eval \"var a=[1,2,3]; a.keys().join(',')\")")
+
 ;; ── Phase 11.arrlike: Array.prototype.* on {length, 0:..., 1:...} ──
 (epoch 3500)
 (eval "(js-eval \"var a = {length: 3, 0: 41, 1: 42, 2: 43}; Array.prototype.slice.call(a).length\")")
@@ -1853,6 +1875,18 @@ check 3607 "true.toString()"               '"true"'
 check 3608 "false.toString()"              '"false"'
 check 3609 "(true).valueOf()"              'true'
 
+# ── Phase 11.arrmore: more Array.prototype methods ────────────
+check 3700 "arr.at(0)"                     '1'
+check 3701 "arr.at(-1)"                    '3'
+check 3702 "arr.flatMap"                   '"1,2,2,4,3,6,4,8"'
+check 3703 "arr.findLast"                  '3'
+check 3704 "arr.findLastIndex"             '4'
+check 3705 "arr.reduceRight"               '"4,3,2,1"'
+check 3706 "arr.toString"                  '"1,2,3"'
+check 3707 "arr.toReversed"                '"2,1,3"'
+check 3708 "arr.toSorted"                  '"1,1,3,4,5"'
+check 3709 "arr.keys"                      '"0,1,2"'
+
 # ── Phase 11.arrlike: array-like receivers on Array.prototype ─
 check 3500 "slice.call arrLike length"     '3'
 check 3501 "slice.call arrLike join"       '"41,42,43"'