Mistune: Path Traversal and XSS in Include directive

The Include directive in src/mistune/directives/include.py does not validate that the resolved file path stays within the source directory, allowing path traversal via ../ sequences to read arbitrary files.

Additionally, error messages and included file contents are rendered without HTML escaping, leading to XSS even when escape=True.

Affected functions:

• Path resolution (lines 27-31)
• block_error() in html.py (line 140)
• render_html_include() (line 72)

The Include plugin must be explicitly enabled via the plugins parameter.