feat: unified diff (patch) output for line mode — discussion

[krkarma777]

Problem

Line mode produces entries/ranges but no standard interchange format. Tooling (code review, CLI, git-adjacent workflows) speaks unified diff.

Proposal (needs scoping discussion first)

• `formatPatch(a, b, options)` → unified diff string with configurable context lines.
• Possibly `applyPatch` later — that doubles the API surface and test burden, so decide deliberately whether it belongs here or stays jsdiff territory.

Constraints

• Keep the core zero-dependency and tree-shakeable (separate export path, e.g. `@krkarma777/string-diff/patch`).
• If we ship it, fuzz round-trip: applyPatch(a, formatPatch(a, b)) === b.