feat: implement box ol and box ul ol decoration styles

[VXNCXNX]

Fixes #214.

What was missing

0d593c2 made decoration styles compose, but box ol and box ul ol were left as TODOs that fell through to a plain box:

DecorationStyle::BoxWithOverline(style) => {
    // TODO: not implemented
    (Box::new(write_boxed), true, style)
}

So --commit-decoration-style='box ol' silently rendered the same thing as box, with no error to tell you the style wasn't supported.

The fix

write_boxed_partial hardcoded ┐ as the top-right corner. It now takes an overline: bool that selects ┐ or ┬─, and that flag threads up through write_boxed_with_horizontal_whisker. The four public draw functions become thin wrappers over two shared bodies, so box/box ol and box ul/box ul ol differ only in that flag rather than in duplicated drawing code.

Result — box ol is the vertical mirror of box ul:

box                 box ul              box ol              box ul ol
──────────────┐     ──────────────┐     ──────────────┬─    ──────────────┬─
commit 94907c │     commit 94907c │     commit 94907c │     commit 94907c │
──────────────┘     ──────────────┴─    ──────────────┘     ──────────────┴─

Tests

You'd already written the expected output for both cases as #[ignore]d tests (test_commit_style_box_ol, test_file_style_box_ol). Those are the spec here — I implemented against them and removed the #[ignore], without touching a single assertion string or width argument. box ul ol had no pre-written test, so I added two modeled exactly on the ol pair.

cargo test:        442 passed; 0 failed; 6 ignored   (was 440 passed; 8 ignored)
cargo fmt --check: clean
cargo clippy:      no new warnings (4 pre-existing in src/utils/process.rs)

What I checked beyond the tests

• No regression on the two styles that already worked. Captured raw output (ANSI codes included) for box and box ul at widths 64/10/1/variable, plus raw and bold modes, before and after the change: 12/12 byte-identical.
• No underflow at narrow widths. box ol never touches line_width; box ul ol reuses the existing guarded line_width - box_width - 1. Ran all four styles at --width=10 and --width=1 against a 46-char commit line, exit 0, no panic.
• Heavy/light stays correct. DOWN_HORIZONTAL is destructured in the same is_bold branch as the other glyphs, so it can't desync. Bold renders ┳.
• --commit-style raw still works with both new styles.

One thing worth your call: the overline whisker is a fixed 2 chars (┬─) regardless of terminal width, since box-family functions ignore line_width. That's what the ignored test asserts, and it matches the existing _line_width: &Width, // ignored precedent, but if you'd rather the overline whisker extend to fill the width like the underline one does, that's a small follow-up.