Highlight unusual Unicode whitespace

[barkure (barkure)]

Summary

• add an opt-in View > Highlight Unusual Whitespace toggle with an Alt+I shortcut
• visualize the 18 non-ASCII, non-control Unicode White_Space characters using width-preserving stand-ins and a yellow background
• keep the setting document-specific and disabled by default, without changing buffer contents or layout
• preserve selection visibility over highlighted whitespace and add English, Simplified Chinese, and Traditional Chinese labels

Motivation

Characters such as no-break spaces, narrow no-break spaces, ideographic spaces, and Unicode line/paragraph separators are otherwise difficult to distinguish from ordinary spacing. This follows up on the invisible-character work discussed in #188 while keeping the additional visualization explicitly opt-in.

Implementation notes

• the existing control-character sanitizer remains the default API and behavior
• the enabled and disabled sanitizer loops are compile-time specialized, so the default hot loop has no per-character unusual-whitespace mode check
• replacements preserve terminal cell width, including the two-cell U+3000 IDEOGRAPHIC SPACE
• no dependencies were added
• in an arm64 macOS release comparison, the executable remained 518,816 bytes and the __text section increased by about 1.2 KiB

Validation

• cargo +nightly fmt --all -- --check
• cargo +nightly test --all-features --all-targets
• cargo +nightly test --workspace --all-targets
• cargo +nightly clippy --workspace --all-features --all-targets -- --no-deps --deny warnings
• cargo +nightly test --workspace --doc
• RUSTDOCFLAGS='-D warnings' cargo +nightly doc --workspace --no-deps
• cargo check --workspace --all-features --all-targets

Tests cover the complete target code-point set, neighboring non-target characters, exact replacement/highlight ranges, width preservation, selection precedence, and the Alt+I toggle path.