Add mix mob.mutate — break the code on purpose and check the tests notice (MOB-153)

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A green suite says the tests ran, not that they guard anything. This has been hand-rolled here roughly seven times and found real defects every time — a record(...) deletable from a frame tracker with 1500 tests green, an -export line whose removal fails load_nif and crash-loops every app at boot, a requested: option whose deletion silently restored the bug it was added to fix.

It also got written wrong once: comparing each mutant against a hardcoded pass-count that was off by one, so every surviving mutation was reported as caught, including a genuine gap. The baseline is now always captured by running, and a baseline that prints no Result: line refuses to continue rather than treating "no output" as a pass.

What the review found — all three demonstrated by running, not reading

The heredoc scanner never closed on """) — the form this task's own Mix.raise("""…""") uses — so it stayed inside the heredoc to EOF and classified 185 of the task module's 226 lines as prose. Every line of the file-rewriting and restore logic was invisible to the tool while the decision record claimed "we ran it on itself".

Worse than the extent is the direction: a desync skips code, so those mutants are never generated, never run, never reported — the output is indistinguishable from a clean bill of health. Opening and closing are now separate tests, and the attribute state has a line budget so a stray bracket in a doc string can't eat a module either.

The docs promised a snapshot that didn't exist. They said the tree was restored "even if the run is interrupted"; the reviewer sent SIGINT mid-run and left the file corrupted. The sentence actively talked the reader out of the one habit that would have saved them. The run now refuses to start unless the target files are clean in git — the only guarantee that survives being killed outright, and it also closes the case where an editor autosaves mid-mutant and restore! overwrites the edit while verifying successfully against its own stale snapshot.

No extension filter, so a branch touching CHANGELOG.md or mix.lock — as this one does — produced a wall of guaranteed survivors, failed, and rewrote mix.lock line by line while running the suite against it.

Two more were this module's own thesis turned back on it: classify/3 defaulted an unmeasurable mutant to killed (now :unmeasured), and from_diff/1 swallowed git's exit status so a typo'd --base measured nothing and reported "outcome": "ok" — the defect this repo fixed in mix mob.deploy one release ago. It also mixed two diffs with different line-number origins and missed staged work entirely; both are now one git diff HEAD.

Dogfooding

Building it found four defects in itself before the review found five more: prose mutation, spec mutation, the heredoc desync, and six untested branches in its own diff parser. Running it on the task module now reports most of run/1 as surviving, which is accurate — this repo's convention is that a Mix task stays a thin unstubbed I/O wrapper with the decisions extracted and tested, so source_file?/1, summary/4, dirty_message/2 and all of MobDev.Mutate are guarded and the orchestration is not. The moduledoc says so rather than letting the report read as a failure.

Also fixes a mint CVE the scanner surfaced (1.9.3 → 1.10.0, lock-only). The two remaining HIGH findings are the known OTP-tarball drift, verified pre-existing on master.

2304 tests, credo and format clean, decision record included.