js-on-sx: arrays accept numeric-string property keys

JS arrays must treat string indices that look like numbers ("0",
"42") as the corresponding integer slot. js-get-prop and js-list-set!
only handled numeric key, falling through to undefined / no-op for
string keys. Added a (and (string-typed key) (numeric? key)) clause
that converts via js-string-to-number and recurses with the integer
key. built-ins/Array: 13/45 → 14/45. conformance.sh: 148/148.

plans/js-on-sx.md

@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-08 — **Arrays accept numeric-string property keys (`arr["0"]`).** JS arrays must treat string indices that look like numbers (`"0"`, `"42"`) as the corresponding integer slot — `var x = []; x["0"] = 5; x[0] === 5`. `js-get-prop` and `js-list-set!` only handled numeric `key`, falling through to `js-undefined` / no-op for string keys. Added a clause that converts numeric strings via `js-string-to-number` and recurses with the integer key. built-ins/Array: 13/45 → 14/45. conformance.sh: 148/148.
+
 - 2026-05-07 — **JS top-level `var` no longer pollutes SX global env; call args use `js-args` to avoid `list` shadow.** `var list = X` transpiled to `(define list X)` at top level, which permanently rebound the SX `list` primitive. Then any later code (including the runtime itself) calling `(list ...)` got "Not callable: <X>". Two-part fix: (1) wrap the whole transpiled program in `(let () ...)` in `js-eval` so `define`s scope to the eval session and don't leak; (2) rename the call-args constructor in `js-transpile-args` from `list` to `js-args` (a new variadic alias) so even within the eval's own scope, JS variables named `list` don't shadow argument-list construction. Array-literal transpile keeps `list` (lists must be mutable). built-ins/Object: 41/50 → 42/50; Array.from on array-likes now works. conformance.sh: 148/148.
 
 - 2026-05-07 — **`Object.__callable__` returns `this` for `new Object()` no-args path.** `js-new-call Object` had `obj.__proto__ = Object.prototype` already set, but then Object.__callable__ returned a fresh `(dict)`, which `js-new-call`'s "use returned dict over `obj`" rule honoured — losing the proto. Added a `is-new` check (`this.__proto__ === Object.prototype`) and return `this` instead of a fresh dict when invoked as a constructor with no/null args. Now `new Object().__proto__ === Object.prototype`, `Object.prototype.isPrototypeOf(new Object())`, and `.constructor === Object` all work. built-ins/Object: 37/50 → 41/50. conformance.sh: 148/148.