chore(cli): mutate only the lines a branch changed #842

Problem

scripts/run-mutation.mjs runs a whole scope. To check a change before pushing, mutation was run by hand five times in one session: the changed hunks of git diff -U0 turned into --mutate file:start-end ranges, and stryker run launched under a throwaway HOME. It is repetitive and easy to get wrong.

Proposal

node scripts/run-mutation.mjs --changed [<base>]: mutate only the lines changed since <base> (default origin/next). Report survivors with file and line, without touching any scope's incremental file or floor.

Acceptance criteria

• On a branch changing one function, only that function's mutants are generated.
• No scope's incremental.json is written.