fix: serialize elements iteratively so deep nesting does not overflow the stack

[zzoraemon043]

Fixes #324

HTMLElement.toString() and innerHTML called each other recursively, so serializing a tree nested a few thousand levels deep threw RangeError: Maximum call stack size exceeded, while parse() handled it fine. Both now go through one iterative serializer with an explicit stack. The output is the same: void tags still go through voidTag.formatNode (so addClosingSlash and custom void tags keep working), other elements get <tag attrs>…</tag>, and text and comment nodes use their own toString().

Added test/tests/issues/324.js (depth 10000 round trip for toString, outerHTML and innerHTML, plus a mixed void tag / comment / attribute case). With npm run test:src the new tests pass; the two Module Import failures are the same with and without this change (they need the built package).