fix(cli): unicode-safe rendering across TUI and commands (3.2.4)

[montfort]

Summary

Follow-up to #52. That PR fixed a UTF-8 panic in wrap_cell_text(); this one removes the byte-vs-visual-column confusion that was scattered across the rest of the CLI — the same anti-pattern that caused the original crash.

All layout sites now route through three new helpers in utils.rs:

• visual_width(&str) -> usize
• truncate_visual(&str, max_cols) -> String
• pad_right_visual(&str, cols) -> String

TUI fixes

• Scrollbar leak: explore's scrollbar no longer shows the document text through its track. The paragraph renders in a reserved body column and the scrollbar owns the rightmost column. ScrollbarState::viewport_content_length is now set, so the thumb has the correct proportional size.
• Line-count estimate: doc_viewer measures line width with UnicodeWidthStr::width, not bytes — scroll position is accurate for CJK/emoji content.
• Navigation tree: date extraction (2026-04-12 → 04-12) is char-safe (no byte slicing on arbitrary created strings); title budget computed in visual columns.
• Metadata panel: related-link truncation uses truncate_visual.
• Status bar: right-aligned padding uses visual width — projects in directories with non-ASCII names now stay aligned.
• Markdown code blocks: alignment fixed for blocks containing CJK / emoji / accented characters.

Command fixes

• validate: NAMING-001, META-004, and the auto-fix path no longer byte-slice the filename date. Non-ASCII where ASCII was expected produces a clean validation error instead of risking a panic.
• analyze: path/function truncation uses visual width with a leading ellipsis; padding is correct under any input.
• status: the Project, Structure, and Documentation tables compute column widths and padding in visual columns.
• new: slug truncation and sequence-number scan use chars().take(N) instead of byte slices.

Dependency change

• unicode-width moved from optional (feature tui) to a direct always-compiled dep. It was already transitive via ratatui, so no new compile cost; promoting it lets non-TUI commands measure visually too.

Test plan

• cargo test — 76 library tests passing (48 pre-existing + 16 markdown regression + 12 new utils::tests::*).
• cargo build --no-default-features — compiles cleanly.
• cargo build --all-features — compiles cleanly.
• cargo build --release — compiles cleanly; devtrail about reports cli-3.2.4.
• cargo clippy --all-features — no new warnings (the 4 remaining are pre-existing in audit_engine.rs, app.rs, index.rs, markdown.rs:flush_spans).
• Manual: ./target/release/devtrail status table borders align correctly.
• Manual: open a doc with CJK content via explore and check nav tree / metadata / code block alignment.
• Manual: confirm the scrollbar no longer shows document text through the track.

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