fix(gate): refuse a byte splice that breaks the JSON it mutates

[CameronBrooks11]

What

tools/mutation_gate.py, _splice: after a byte splice into a .json target, the result is passed to json.loads. If it does not parse, _splice raises MutantError("mutated JSON does not parse: ..."). apply already hands a MutantError back to main, which scores the mutation BROKEN and fails the run. The docstring now says this check exists, and that any other extension is spliced unchecked.

tests/test_mutation_gate.py:

• test_a_byte_splice_that_breaks_the_json_is_refused_not_scored splices "required": ["kind",, into the schema and requires MutantError.
• test_every_json_mutation_in_the_gate_still_parses, parametrised over every .json mutation in MUTATIONS, covers the other direction: the new refusal must not reject any real mutation.

One new mutation, "a byte splice that breaks the JSON is scored instead of refused", disables the check.

Why

A token splice is compiled; a byte splice was parsed by nothing. A mutation that only broke the file's syntax made pytest fall over on a parse error, and the gate scored that as a kill of a change nothing had detected.

Testing

Red first, on main (bf716e3). I put the issue's mutation through the real gate (MUTATIONS replaced by that one entry, main() called) in a throwaway worktree, because the gate writes in place:

  caught    syntax break in the schema  [9dc95205726c -> 93b44a35270c]
1 mutations, 1 caught, 0 survived, 0 broken
gate exit 0

The new unit test on main:

$ uv run pytest -q tests/test_mutation_gate.py
E       Failed: DID NOT RAISE MutantError
1 failed, 6 passed

Same one-entry gate run, on this branch:

  BROKEN    syntax break in the schema  [mutated JSON does not parse: Expecting value: line 8 column 23 (char 626)]
1 mutations, 0 caught, 0 survived, 1 broken
gate exit 1

The valid direction: all four real schema mutations still splice to parseable JSON and are still caught.

$ just check
All checks passed!
$ just test
150 passed
$ just mutants        # in a throwaway detached worktree of this commit
  caught    'kind' stops being required
  caught    'kind' may be any JSON scalar
  caught    'kind' stops having to be lower-kebab
  caught    case-body keys stop having to be snake_case
  caught    a byte splice that breaks the JSON is scored instead of refused
100 mutations, 100 caught, 0 survived, 0 broken

The count is main's 99 plus the one new mutation, and the 99 are all still caught. The issue's "73" was the count when it was filed. I also applied the new mutation with the gate's hash-checked apply and ran its listed test on its own: exit 1, and the file restored to its original hash.

The issue's last bullet asks for _splice's docstring to be corrected. That correction is already on main: since #18 the docstring says a layout-sensitive pattern is "loud rather than silent" and scored BROKEN. This PR only adds the paragraph about checking the splice.

Interaction with #19: #19 adds a byte-path mutation of .pre-commit-config.yaml. Under this change, YAML is spliced unchecked, as the docstring now says, because the stdlib has no YAML parser. That mutation keeps the file valid YAML and its test reads the file with a regex, so it is unaffected.

Follow-up (review): the parse check could skip itself

test_every_json_mutation_in_the_gate_still_parses is parametrised over MUTATIONS filtered to .json paths. With an empty filter, pytest skips the test and exits 0. Shown in a throwaway worktree, with the filter changed to .nothing:

SKIPPED [1] tests/test_mutation_gate.py:128: got empty parameter set for (mutation)
1 skipped, 3 deselected
exit=0

• The filtered list is now a module constant, JSON_MUTATIONS. A plain, non-parametrised test_there_are_json_mutations_to_check requires at least 4 entries, the schema mutations the gate carries today. With the same emptied filter it fails: FAILED ...test_there_are_json_mutations_to_check, 1 failed, 3 passed, 1 skipped.
• New mutation "the JSON mutations the parse check covers filter down to none". It empties the filter, and its listed test exits 1 on its own (gate apply, hash-checked, file restored).

With main (#21) merged in, at 5ee2d6f:

$ just check
All checks passed!
$ just test
158 passed
$ just mutants        # throwaway detached worktree
  caught    a byte splice that breaks the JSON is scored instead of refused
  caught    the JSON mutations the parse check covers filter down to none
109 mutations, 109 caught, 0 survived, 0 broken

After merging main with #19 (merge commit 3404ef9): just check all checks passed; just test 159 passed; just mutants (throwaway detached worktree) 110 mutations, 110 caught, 0 survived, 0 broken. CI green.

Closes #10