URLSearchParams II

Requirements

• Export SearchParams as named and default. Preserve the first version’s string constructor, get, getAll, has, readonly size, entries, keys, values, default iteration, forEach, and toString. Inputs to named methods are strings; nonstring coercion is outside scope. Do not use native URLSearchParams or URL.
• Parse query strings after removing at most one leading ?. Split on &, ignore empty segments, and split each entry on its first =. Missing = means an empty value. Decode + as a space before percent-decoding. Preserve duplicate/empty keys and order. Valid Unicode is in scope; malformed percent encoding throws URIError, deliberately differing from native recovery.
• append(name, value) adds a decoded pair at the end. set(name, value) replaces the first matching pair in its existing position and removes later pairs with the same name; append if absent. delete(name) removes every matching key; delete(name, value) removes only exact name/value matches. Optional value === undefined means match by name alone. has(name, value) optionally checks a matching pair.
• sort() stably orders pairs by keys using lexicographic UTF-16 code-unit order, not locale order or values. Equal-key pairs retain their relative order. size always reflects pair count. Mutation methods return undefined.
• Iterators and forEach are live and use an increasing numeric index into the current list. Appended pairs can be visited; deleting an earlier/current pair may shift and skip the next pair. Replacing or sorting affects subsequent visits. Each yielded entry is a fresh pair, so changing it cannot mutate storage. Avoid unbounded appending inside iteration.
• getAll returns a fresh array. forEach calls callback(value, key, instance) with thisArg as its receiver. toString emits no leading ?, encodes spaces as +, leaves ASCII letters/digits/*/-/./_ unescaped, and uses uppercase UTF-8 percent encoding for other characters.
• No object/iterable constructors, URL parsing, lone-surrogate recovery, case folding, nested-query syntax, or browser location mutation is required. Method keys/values are already decoded; do not decode them again when appending or setting.

Examples

Input: const p = new SearchParams("a=1&b=2&a=3"); p.set("a","x"); p.append("a","y"); p.toString();
Output: a=x&b=2&a=y

Input: p.delete("a","x"); p.sort(); p.toString();
Output: a=y&b=2