Reject unmatched closing tags at document level

[Vryuz]

Inputs such as </data>, an XML declaration followed by </data>, or <data/> followed by </data> currently return XML_SUCCESS. When ParseDeep encounters a closing element without a parent end-tag recipient, report XML_ERROR_MISMATCHED_ELEMENT at that closing tag's line and return failure. Nested closing-tag transfer and node cleanup stay unchanged.

Fixes #972.

Regression coverage includes closing tags after a declaration, a root, text, and comments, plus document cleanup/reuse and accepted fragment controls. The older #291 crash test uses the malformed input &#0</a>; it now expects a mismatch and still calls Print. A separate accepted <a>&#0</a> control retains coverage of lazy numeric-reference processing during printing.

Validation on Ubuntu with GCC 13.3:

• Final tests against the original parser: 546 checks pass, 24 fail.
• Debug and Release CTest runs: 570 checks pass, 0 fail in each.
• Full ASan/UBSan CTest run with leak detection enabled: 570 checks pass, no findings.

The behavior change is limited to unmatched document-level closing tags. Existing supported text/multiple-element fragments remain accepted; no public API or ABI changes are introduced.