Add artprompt transform for ASCII-art word masking.

[panditdhamdhere]

Summary

Implements the artprompt transform from the improvement roadmap (Phase 0).

Based on ArtPrompt (Jiang et al., ACL 2024), the transform:

1. Picks a likely trigger word (prefers terms from the existing NEUTRAL list, otherwise the longest substantive word)
2. Masks it in the prompt with ?
3. Appends a 5-line block-letter ASCII art rendering of that word

This lets the harness evaluate vision-in-text jailbreak resistance via parseltongue / query_target transform chains.

Changes

• wallbreaker/transforms/ascii_art.py — new artprompt_encode() implementation
• wallbreaker/transforms/__init__.py — register as one-way, lossy transform
• tests/test_transforms.py — unit test for masking + ASCII art output

Test plan

• pytest tests/test_transforms.py -q (76 passed)
• pytest tests/test_transforms.py tests/test_target_encode.py tests/test_extra_features.py -q (94 passed)
• Full suite (pytest -q) on maintainer CI

Notes

• Technique is attributed to the ArtPrompt paper; no novelty claimed.
• Uses a built-in block font (no new dependencies).
• Masks one trigger word per call, matching the paper's per-masked-prompt pattern.