* feat(ci): mutation testing PR-time lane + cargo xtask mutants-pr (#182 PR 3)

Adds a label-gated PR-time mutation lane and the xtask wrapper it
invokes, keeping mutation off every PR's hot path while making it cheap
to opt in when warranted.

  cargo xtask mutants-pr --changed [--base origin/main] [--dry-run]

is a thin wrapper around `cargo-mutants` that:

  1. Refuses to run without `--changed`. Whole-workspace mutation is
     intentionally not part of the PR-time lane — that lives in the
     weekly schedule (see docs/ci/test-evidence-lanes.md).
  2. Computes `git diff <base>...HEAD --name-only -- '*.rs'`, filters
     out `tests/` and `benches/` paths (cargo-mutants only mutates
     production source).
  3. Runs `cargo mutants --no-shuffle --file <each-path>` (or `--list`
     when `--dry-run` is passed).
  4. If `cargo-mutants` is missing locally, prints install instructions
     and exits advisory-success rather than erroring. CI installs the
     tool before invoking.
  5. Surfaces `cargo mutants` non-zero exit codes — surviving mutants
     are load-bearing failures, unlike ripr's purely advisory findings.

`.github/workflows/mutation.yml` adds the `mutants-pr` job alongside
the existing `mutants-weekly` job. The PR job's gate:

  if: github.event_name == 'pull_request' && (
    contains(github.event.pull_request.labels.*.name, 'mutation') ||
    contains(github.event.pull_request.labels.*.name, 'full-ci')
  )

The weekly job's crate list stays unchanged (shipper-duration,
shipper-types, shipper-config). Expanding it to the full trust-critical
surface (shipper-core, shipper-encrypt, etc.) is too expensive for a
60-minute job and is its own future rollout step; the policy stays
about routing, not coverage scope.

  xtask/src/mutants.rs          new — 3 unit tests
  xtask/src/main.rs             `mutants-pr` subcommand wiring
  .github/workflows/mutation.yml   two-job split (mutants-weekly +
                                   mutants-pr)
  docs/ci/test-evidence-lanes.md   advisory-routed table + targeted
                                   mutation section updated
  CHANGELOG.md                  Added entry

Tests:

  mutants::tests::install_hint_mentions_cargo_install    (new)
  mutants::tests::args_defaults_are_explicit             (new)
  mutants::tests::changed_requires_the_flag              (new)

24/24 tests passing.

Refs #182 (this is the third PR in the #182 sequence: advisory lane in
#230, --base + projection in #231, badges in #232, mutation scoping
here).

* fix(xtask): wrap mutants-pr doc-comment Behaviour block in fenced code

`<REF>` inside a doc comment is parsed by rustdoc as an invalid HTML tag
under `-D rustdoc::invalid-html-tags` (implied by RUSTDOCFLAGS=-D
warnings on the CI Documentation job). Wrap the whole Behaviour block in
a ```text fence so the angle brackets are literal.