zbateson/mail-mime-parser has uncontrolled resource consumption (CPU/memory DoS) parsing untrusted MIME

### Impact An uncontrolled resource consumption / algorithmic complexity vulnerability affecting any application that parses untrusted email with this library. Three independent parsing paths are super-linear in cost, so a byte-size cap on the caller side does **not** bound the work done. A crafted message under 2 MB can consume seconds of CPU or hundreds of megabytes to multiple gigabytes of memory (leading to an out-of-memory kill), enabling denial of service. The parse is lazy, but the cost is paid on the first `getAllParts()` or content read. ### Details 1. **Deep multipart nesting is O(depth²).** `MimeParserService::findContentBoundary()` tests each content line against the current part and every ancestor via the recursive `ParserMimePartProxy::setEndBoundaryFound()`, so a nesting depth of D costs 1 + 2 + … + D comparisons. A ~600 KB message nested ~10,000 deep does not finish parsing in a minute. 2. **Many sibling parts is O(n²).** `PartChildrenContainer::add()` appends each child with `array_splice($children, count($children), 0, [$part])`, which reindexes the entire array on every call. The same container backs UUEncoded `begin` parts via `NonMimeParserService`. 3. **Header buffering is unbounded.** `HeaderParserService::parse()` reads header lines up to a blank line with no limit on header count or total size, so a few megabytes of header lines can hold hundreds of megabytes resident; a ~5 MB message of headers can reach multiple gigabytes and be OOM-killed. ### Proo