js-on-sx: delete obj.key actually removes the key

js-delete-prop was setting value to js-undefined instead of
removing the key, so 'key' in obj remained true and proto-chain
lookup didn't fall through. Switched to dict-delete!.
Now delete Boolean.prototype.toString; Boolean.prototype.toString()
walks up to Object.prototype.toString and returns "[object Boolean]".
built-ins/Boolean: 21/27 → 22/27. 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 — **`delete obj.key` actually removes the key.** `js-delete-prop` was setting the value to `js-undefined` instead of removing the key, so subsequent `'key' in obj` returned true and proto-chain lookup didn't fall through to the parent. Switched to `dict-delete!` (existing SX primitive). Now `delete Boolean.prototype.toString; Boolean.prototype.toString()` correctly walks up to `Object.prototype.toString` and returns `"[object Boolean]"`. built-ins/Boolean: 21/27 → 22/27. conformance.sh: 148/148.
+
 - 2026-05-08 — **`Boolean(NaN) === false` (and `!NaN === true`).** `js-to-boolean` was returning `true` for NaN because NaN ≠ 0 by IEEE semantics, so the `(= v 0)` test fell through to the truthy-else clause. Per ES, NaN is one of the falsy values. Added a `(js-number-is-nan v)` clause. built-ins/Boolean: 19/27 → 21/27. conformance.sh: 148/148.
 
 - 2026-05-08 — **Global `eval(src)` actually evaluates the source.** Was returning the input string unchanged: `eval('1+2')` returned `"1+2"`, not `3`. Per spec, `eval(string)` parses and evaluates as JS; non-string input passes through. Wired the runtime stub through `js-eval` (which already does the lex/parse/transpile/eval pipeline) when the arg is a string. Fixes `String(eval('var x'))`, the harness internal `eval(...)`, and any test that calls `eval` for runtime evaluation. built-ins/String fail count: 13 → 11. conformance.sh: 148/148.