js-on-sx: js-num-from-string uses string->number for exp-form

Was computing m * pow(10, e) for "1.2345e-3" forms; floating-point
multiplication introduced rounding (Number(".12345e-3") -
0.00012345 == 2.7e-20). The SX string->number primitive parses the
whole literal in one IEEE round, matching JS literal parsing. Falls
back to manual m * pow(10, e) only when string->number returns nil.
built-ins/Number: 42/50 → 43/50. 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.
 
+- 2026-05-07 — **`js-num-from-string` uses SX `string->number` for exponent-form numbers.** Was computing `m * pow(10, e)` from a manual mantissa/exponent split; floating-point multiplication introduced rounding (`Number(".12345e-3") - 0.00012345 == 2.7e-20`). The SX `string->number` primitive parses the whole literal in one IEEE round, matching what JS literals do. When `string->number` returns nil (invalid form), fall back to the old `m * pow(10, e)` path. built-ins/Number: 42/50 → 43/50. conformance.sh: 148/148.
+
 - 2026-05-07 — **Constructors (`Object`/`Array`/`Number`/`String`/`Boolean`) carry `__proto__ = Function.prototype`.** Per spec, the constructors are functions and inherit from `Function.prototype`, so `Function.prototype.foo = 1; Array.foo === 1`. Previously the constructor dicts had no `__proto__`, so they only saw `Object.prototype` via the recent fallback — `Function.prototype` mutations were invisible. Added a `(begin (dict-set! ...))` post-init at the end of `runtime.sx` after the constructors are defined. Combined with the existing Object.prototype fallback, the proto chain now terminates correctly for the constructor → `Function.prototype` → `Object.prototype` walk. built-ins/Number: 41/50 → 42/50, built-ins/String: 75/99 → 78/99, built-ins/Array: 12/45 → 13/45. conformance.sh: 148/148.
 
 - 2026-05-07 — **`js-neg` preserves IEEE-754 negative zero.** `-0` was returning `0` (rational integer) because `js-neg` did `(- 0 (js-to-number a))`, which loses sign-of-zero in any arithmetic implementation that follows IEEE 754. Per JS spec, `-0` and `1/-0 === -Infinity` must be observable. Switched to `(* -1 (exact->inexact (js-to-number a)))` so the result is always a float and `-0.0` is preserved. Fixes `Math.asinh(-0)` and other `-0`-sensitive tests; `1/(-0) === -Infinity` now works. built-ins/Math: 41/45 → 42/45. conformance.sh: 148/148.