runtime: String(new Map()) renders "[object Object]", not "[object Map]" — the ToString fallback ignores Symbol.toStringTag

[proggeramlug]

Summary

String(x) on a builtin exotic with a Symbol.toStringTag renders "[object Object]" instead of "[object <Tag>]".

Repro

console.log(String(new Map()));            // node: [object Map]      perry: [object Object]
console.log(String(new Set()));            // node: [object Set]      perry: [object Object]
console.log(String(Promise.resolve(1)));   // node: [object Promise]  perry: [object Object]
console.log(`${new Map()}`);               // node: [object Map]      perry: [object Object]

Why

Object.prototype.toString is spec'd to read [[Get]](O, @@toStringTag) and use it when it is a string (ES2024 §20.1.3.6); Map.prototype[@@toStringTag] is "Map", Set's is "Set", Promise's is "Promise". Perry's ToString path (js_jsvalue_to_string in crates/perry-runtime/src/value/to_string.rs) ends in a hardcoded "[object Object]" fallback that never consults the tag. Note the runtime does have a to_string_tag.rs — it just isn't wired into this fallback.

An OWN toString on these instances is already honoured correctly (m.toString = () => "X"; String(m) → "X"), so this is purely the default-tag rendering.

Found while fixing #6370 (own toString overrides ignored on the ToString coercion path for RegExp/Date); pre-existing on main.