test(x11): cover INCR clipboard boundaries

[xxx]

Closes #375.

Adds X11 integration coverage for both unsupported incremental-transfer boundaries:

• a foreign owner returning an INCR property is rejected explicitly rather than read as truncated text
• text larger than Xvfb's 16 MiB BIG-REQUESTS ceiling is refused rather than leaving the requestor unanswered or returning partial data

The investigation also corrected the issue's approximate 256 KiB boundary: Xvfb accepts a 1 MiB one-shot property write through BIG-REQUESTS, so the outbound test uses 20 MiB to cross the actual server limit.

Verification:

• both new ignored X11 integration tests directly
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings