fix(cli): code-block bg fragments on narrow panels (cli-3.4.1)

[montfort]

Summary

• Fix the gray background of fenced code blocks in devtrail explore breaking into truncated stripes when the document panel is narrower than the longest code line (visible on terminal resize and certain initial layouts — reported via screenshot).
• Root cause: each code line was padded to the block's longest line and emitted as a single styled Line. When that Line exceeded the panel width, ratatui's Paragraph::wrap re-flowed it, dropping trailing styled whitespace at the wrap point and leaving visible gaps between content rows.
• Fix: pre-wrap inside markdown_to_lines using a new visual-column-aware hard-wrap helper (wrap_visual_columns). Each emitted Line is now bounded by available_width, so ratatui never re-wraps it and the gutter background paints uniformly on every visual row.
• UTF-8 / CJK safe (double-wide chars never split mid-glyph). Indentation is preserved (no whitespace trim).
• Patch bump: cli-3.4.0 → cli-3.4.1.

Test plan

• cargo test — 123 unit tests + 86 integration tests pass; 7 new regression tests added (wrap_visual_columns_* + code_block_wraps_within_panel_width + code_block_blank_lines_keep_background).
• cargo build --release — clean build as devtrail-cli v3.4.1.
• Manual TUI smoke test: open devtrail explore on the C4-DIAGRAM-GUIDE.md from the screenshot's narrow window. Code blocks should now have a uniform gray background regardless of resize. Blank lines inside code blocks must stay gray.
• Verify wide-panel rendering (image feat: rebrand to Strange Days Tech + CLI scaffolder + repo restructure #1 in the report) is unchanged: gray bar still ends at the longest content + 4 cols, not full panel width.

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