runtime: complete String.prototype.matchAll iterator and validation semantics #2845

Summary

String.prototype.matchAll is implemented enough to return a materialized array of capture arrays, but it does not match Node's observable matchAll contract. Node returns an iterator, rejects non-global RegExp arguments, accepts string/non-RegExp patterns by constructing a global RegExp, starts from the RegExp's lastIndex, and each match result exposes metadata such as index, input, and groups.

This is separate from closed #317 / PR #331, which fixed the earlier hard codegen failure for basic matchAll use.

Expected Node behavior

On Node v25.9.0:

Array.from("a1b2".matchAll(/(\d)/g), m => [m[0], m[1], m.index]);
// [["1", "1", 1], ["2", "2", 3]]

"a".matchAll(/a/);
// TypeError: String.prototype.matchAll called with a non-global RegExp argument

Array.from("a.a".matchAll("."), m => [m[0], m.index]);
// [["a", 0], [".", 1], ["a", 2]]

const re = /a/g;
re.lastIndex = 2;
const seen = Array.from("a_a".matchAll(re), m => m.index);
// seen === [2]
// re.lastIndex is still 2

Current Perry evidence

Current origin/main has a runtime helper, but it is a simplified array materializer:

• crates/perry-codegen/src/lower_string_method.rs lowers str.matchAll(arg) by unboxing arg as a regex handle and calling js_string_match_all(str, re).
• crates/perry-runtime/src/regex.rs::js_string_match_all() returns *mut ArrayHeader, not an iterator object.
• If the receiver or regex pointer is invalid, js_string_match_all() returns an empty array instead of applying RegExpCreate semantics for string/non-RegExp arguments or throwing for invalid non-global RegExp input.
• The helper uses regex.captures_iter(str_data) over the whole string, with no visible check for the g flag and no use of a RegExp lastIndex starting point.
• Inner match arrays store only full match and capture-group slots. There is no visible population of Node's index, input, or groups properties.

Suggested test surface

Add parity tests for:

• "a".matchAll(/a/) throwing TypeError.
• Array.from("a.a".matchAll("."), m => [m[0], m.index]) matching Node's string-pattern behavior.
• const re = /a/g; re.lastIndex = 2; Array.from("a_a".matchAll(re), m => m.index) starting from index 2 without mutating re.lastIndex.
• match result metadata: m.index, m.input, m.groups, and capture group values.
• iterator shape: typeof iter.next === "function" and results are produced lazily through .next().

Scope / non-goals

This issue is only for String.prototype.matchAll argument validation, iterator shape, start-index behavior, and match-result metadata. Broader RegExp flag execution gaps such as sticky/dotAll/unicode/indices are tracked separately by #2828.