Fix panic when wrapping a wide char with a background at tiny width

[nagendramohan]

bat aborts with 'capacity overflow' running e.g.

printf '📦📦\n' | bat --terminal-width 1 --wrap character \
    --highlight-line 1 --color always

When a character is wider than the terminal (a double-width CJK char or emoji) and a background is painted on the line, the per-line cursor advances past cursor_max, and the background fill computes " ".repeat(cursor_max - cursor) — a usize subtraction that underflows and aborts inside str::repeat.

Clamp the fill width with saturating_sub so a terminal narrower than a single glyph yields an empty fill instead of underflowing, mirroring the clamping bat already applies elsewhere.

Adds a regression test (verified failing before the fix) and CHANGELOG entry. Closes #3844.

Fixes #3844.

bat aborts with capacity overflow on e.g.:

printf '📦📦\n' | bat --terminal-width 1 --wrap character --highlight-line 1 --color always

When a character is wider than the terminal (a double-width CJK char or emoji) and a background is painted on the line, the per-line cursor advances
past cursor_max, so the background fill " ".repeat(cursor_max - cursor) (src/printer.rs) does a usize subtraction that underflows and aborts inside
str::repeat.

Fix: clamp the fill width with saturating_sub, so a terminal narrower than a single glyph yields an empty fill instead of underflowing — mirroring the clamping bat already applies elsewhere. Same defect class as the --style=snip width-1 panic (#3803).

Adds a regression test (verified failing before the fix) and a CHANGELOG entry. cargo fmt --check, cargo clippy, and the full integration suite (255 tests) all pass locally.