Add opt-in DTD processing behind Limits(allowDoctype:), off by default. When on,
DoctypeScanner reads the <!DOCTYPE> internal subset and honors internal
<!ENTITY name "value"> general entities, which EntityDecoder expands recursively
in text and attribute values.
Safety is mandatory and reimplemented from the Expat amplification-accounting
model (no upstream code copied):
- only entity-expanded characters count against a shared budget
(Limits.maxEntityExpansion); overflow raises amplificationLimitExceeded, which
defeats billion-laughs.
- a visiting set detects recursive entities (recursiveEntity).
- undeclared references raise undefinedEntity.
- external entities are never stored or loaded, so XXE stays closed.
The default still rejects <!DOCTYPE>. 8 DTD tests (incl. billion-laughs and the
external-entity refusal); 49 total.
Add PureXML.Parsing.Limits: configurable caps on nesting depth (default 256,
matching libxml2), name length, and content length, enforced during scanning in
the event reader. This protects against pathological input (deeply nested or
oversized documents) and keeps the recursive Model.Node value safe to build and
tear down (extreme depth otherwise overflows on ARC release). Threaded through
Parser.parse and the top-level parse/events API. Reimplemented from libxml2's
bounded-by-default posture; no upstream code copied.
