Emit a unified diff per surviving mutant (Vimes parity)

[JohnCobbler]

Vimes.jl wrote a .diff per surviving patch. Gremlins reports coordinates (relpath:line, operator, original -> replacement) but not the actual patch, so you cannot see the mutant at a glance.

We already hold everything needed: the site byte_range, original, and replacement. Rendering a one-hunk unified diff per survivor is cheap.

Scope. Add diff rendering for surviving mutants — a --diff CLI flag and/or a section in the report. Deterministic, sorted (match the existing survivor ordering).

Good first issue: self-contained, no subprocess work — pure rendering over existing MutantResult / MutationSite data. See src/report.jl for the current survivor section.

[JohnCobbler]

Done in f657e36. render_survivor_diffs emits a unified-diff hunk per surviving mutant — the original line as -, the mutated line as +, under a @@ header — exposed via a --diff CLI flag and the public API. Tested in test/test_diff.jl; merged to master.