Underline the changed parts of a renamed path

[VXNCXNX]

Fixes #963.

Disclosure per AI_POLICY.md: written with AI assistance (Claude Code). I've read every line and am accountable for it; happy to discuss any part.

You asked to underline the changed parts of the path in a rename header. This does that, segment by segment on /:

same dir, new name    Renamed from src/[a.ts] to src/[b.ts]
same name, new dir    Renamed from [old]/a.ts to [new]/a.ts
both changed          Renamed from [old]/[a.ts] to [new]/[b.ts]
extra segment         Renamed from scripts/backup-db.ts to [src]/scripts/backup-db.ts

(brackets = underlined). The last one is the case from your issue: the left side is entirely unchanged, so only the added src is marked.

The one design question

extra_info is a plain String, so underlining meant either embedding codes or changing the type. I kept the String and embedded, gated on the already-computed use_color. Changing the type would have rippled through summary.rs, main.rs (~10 sites), inline.rs, side_by_side.rs and style.rs — far more than this issue warrants. As done, exactly one call site changed.

One subtlety worth flagging: style.rs:569 dims the whole line, so the underline is closed with \x1b[24m (underline-off) rather than \x1b[0m, which would have killed the dim for the rest of the line. That's also why owo-colors' Style wasn't usable here — it emits a full reset.

--color never returns the plain strings untouched, verified with cat -v for all four shapes and pinned by a test.

cargo test         124 passed (unit), 23 passed (integration)
cargo fmt --check  clean

Tests cover the four shapes above plus the no-colour case.