#78 highlight: O(n²) per-char suffix allocation makes builds take minutes

What

src/highlight.rs:252 rebuilds the rest of the line for every character:

let rest: String = chars[i..].iter().collect();

That is O(n²) allocation/copy per line, and highlight_lines runs for every text file up to MAX_HIGHLIGHT_BYTES = 512 KiB.

Reproduced

Repo with one 64 KB single-line JS file ('var x=1;' * 8000):

hu13 build /tmp/hlprobe -o /tmp/hlout
real 0m27.5s

Quadratic scaling means a 512 KiB minified file can take ~30 minutes; several such files stall a build.

Impact

Build-time DoS / unacceptable builds on ordinary minified assets. MAX_HIGHLIGHT_BYTES does not protect because the cost is per line.

Fix

• Scan on byte/char indices of &str (or one Vec<char> per line) without building suffix Strings.
• Consider capping a single line (e.g. 8 KiB) to plain text; keep whole-file cap.
• Add a timing/scaling unit test with a 64 KB single-line fixture (coarse bound, not wall-clock tight).