CPU renderer: opening a complex HTML email terminates the client

[iamdarkle]

Opening the attached email while using the CPU — Low Memory renderer
terminates the entire desktop client instead of displaying the message or
falling back to plain text.

Steps:

1. Use the CPU renderer.
2. Select the message.
3. The application process exits.

Expected:
The message renders normally, or the reader switches to plain text if its HTML
cannot be rendered safely.

Notes:

• The file (test) is approximately 381 KB.
• Its sanitized HTML is approximately 200 KB and contains roughly 6,000 DOM nodes.
• Parsing, layout, and bounded CPU tile rendering pass in an isolated regression test.
• The failure occurs only in the complete application, suggesting a native abort,
  resource failure, or integration/lifecycle problem outside the recoverable Rust
  renderer error path.
• A crash stack trace or stderr output is needed to identify the failing native call.