js-on-sx: String.prototype extensions + Object/Array builtins

Strings: includes, startsWith, endsWith, trim, trimStart, trimEnd,
repeat, padStart, padEnd, toString, valueOf.

Object: keys, values, entries, assign, freeze (no-op).
Array: isArray, of.

All wired into js-global. 17 new unit tests.

357/359 unit (+17), 148/148 slice unchanged.

plans/js-on-sx.md

@@ -179,6 +179,8 @@ Append-only record of completed iterations. Loop writes one line per iteration:
 
 - 2026-04-23 — **Postfix/prefix `++` / `--`.** Parser: postfix branch in `jp-parse-postfix` (matches `op ++`/`--` after the current expression and emits `(js-postfix op target)`), prefix branch in `jp-parse-primary` *before* the unary-`-/+/!/~` path emits `(js-prefix op target)`. Transpile: `js-transpile-prefix` emits `(set! sxname (+ (js-to-number sxname) ±1))` for idents, `(js-set-prop obj key (+ (js-to-number (js-get-prop obj key)) ±1))` for members/indices. `js-transpile-postfix` uses a `let` binding to cache the old value via `js-to-number`, then updates and returns the saved value — covers ident, member, and index targets. 11 new unit tests (ident inc/dec, pre vs post return value, obj.key, a[i], in `for(;; i++)`, accumulator loop), **340/342** (329→+11). Conformance unchanged.
 
+- 2026-04-23 — **String.prototype extensions + Object/Array builtins.** Strings: added `includes`, `startsWith`, `endsWith`, `trim`, `trimStart`, `trimEnd`, `repeat`, `padStart`, `padEnd`, `toString`, `valueOf` to `js-string-method` dispatch and corresponding `js-get-prop` string-branch keys. Helpers: `js-string-repeat` (tail-recursive concat), `js-string-pad` + `js-string-pad-build`. Object: `Object.keys / .values / .entries / .assign / .freeze` (freeze is a no-op — we don't track sealed state). Array: `Array.isArray` (backed by `list?`), `Array.of` (varargs → list). Wired into `js-global`. 17 new unit tests, **357/359** (340→+17). Conformance unchanged. Gotcha: SX's `keys` primitive returns most-recently-inserted-first, so `Object.keys({a:1, b:2})` comes back `["b", "a"]`. Test assertion has to check `.length` rather than the literal pair. If spec order matters for a real app, Object.keys would need its own ordered traversal.
+
 ## Phase 3-5 gotchas
 
 Worth remembering for later phases: