docs: the MSRV holds for a default build and not with `vault`

`cargo +1.88 check --all-targets` now passes, so `slice::as_chunks` in `da769bf`
is built at the declared floor rather than inferred from a lint. That gap is
closed. CI is green on all three platforms.

Running the same check with `--all-features` is not green, and the reason is a
dependency rather than this crate: ironvault@7.0.0 requires rustc 1.89. So
`rust-version = "1.88"` is true for a default build and false with the optional
`vault` feature enabled.

Nothing in the project checks that combination. CI builds `--all-features` on
stable; the documented MSRV check builds 1.88 on default features. The only
combination that fails is the one neither runs -- which is the same gap the
*Feature combinations* job exists to close, one level down in the dependency
graph. The command is added to the verification list marked as failing.

Not fixed here on purpose. `rust-version` is one value per package and cannot
express "1.89 with vault", so the choices are raising the floor, pinning
ironvault, or documenting the exception -- a decision about which users get
dropped, which belongs to the maintainer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>