js-on-sx: String.prototype.* ToString-coerces non-string this; .call/.apply skip global-coerce for built-ins

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.
 
+- 2026-05-10 — **`String.prototype.*` ToString-coerces non-string/non-undef this; `.call` / `.apply` skip global-coercion for built-in callables.** `String.prototype.trim.call(false)` was returning `"[object Object]"` because (a) `.call`/`.apply` blanket-coerced null/undefined `thisArg` to `js-global-this`, swallowing the original null, and (b) `js-string-proto-fn` fell back to `"[object Object]"` for any non-string this. (1) `js-string-proto-fn` now ToString-coerces primitive thisVal and raises TypeError for null/undefined (matches `RequireObjectCoercible` semantics for built-in String methods). (2) New `js-call-this-coerce` helper applies the legacy `js-coerce-this-arg` only when `recv` is a user lambda/component; built-in dict-with-`__callable__` methods get the raw `thisArg` (so they can see and reject null/undefined themselves, or accept primitive thisArgs without ToObject). Result: built-ins/String/prototype/trim 7/30 → 30/30 (+23). Function/prototype/apply 10/30 → 21/30. expressions/array 21/30 → 22/30. conformance.sh: 148/148.
+
 - 2026-05-10 — **`**` / `Math.pow` honour JS spec edge cases for NaN, ±0, abs(base)=1+Infinity, plus `Number.prototype.valueOf` accepts ignored args.** (1) New `js-pow-spec` shared by `js-pow` (operator) and `js-math-pow`: NaN exponent → NaN, exponent 0 → 1 (even with NaN base), NaN base + non-zero exp → NaN, abs(base)=1 with exp=±Infinity → NaN. Underlying `pow` handles the rest. (2) Number.prototype.valueOf was `(fn () ...)` and rejected the spec-allowed extra arg with "lambda expects 0 args, got 1"; now `(fn (&rest args) ...)`. Result: language/expressions/exponentiation 23/30 → 25/30 (+2). built-ins/Math/pow 27/27 holds. conformance.sh: 148/148.
 
 - 2026-05-10 — **`Number.prototype.toString(radix)` no longer crashes on rational division-by-zero.** `js-num-to-str-radix` was probing for ±Infinity by comparing against `(/ 1 0)` / `(/ -1 0)` — but on the rational arithmetic path that throws "rational: division by zero" before the comparison ever happens, so every `Number(x).toString(radix)` call exploded. Replaced the probes with `(js-infinity-value)` / `(- 0 (js-infinity-value))` and the NaN check with `js-number-is-nan`. Result: built-ins/Number/prototype/toString 0/30 → 29/30 (+29). Number 26/30. conformance.sh: 148/148.