build: raise the MSRV to 1.89, which is true of every feature combination

1.88 held for a default build and not with `vault`: `ironvault` requires 1.89
and all fourteen of its published versions do, so pinning back was never an
option. The choice was between claiming 1.88 with a footnote or claiming what is
true everywhere.

A `rust-version` meaning "1.88, except…" is the same shape of defect as an
ontology entity declaring a provenance stronger than a row can carry -- the
reader has to already know the exception to avoid being misled. One value has to
cover every combination, so it declares the weakest floor that holds for all of
them.

Verified by building: `cargo +1.89 check --all-features --all-targets` passes.
That replaces two commands in the verification list, one of which was expected
to fail -- and this project has already learned what happens to a check that
always returns the same answer.

Also brings the handoff current: the branch is pushed and green, and the
`memory.bandwidth` "loose thread" is recorded as what it turned out to be. A
derived value resolving while its declared input is absent was read here as the
schema being too strict; it was the code inventing numbers. Worth keeping as a
lesson about which reading of ambiguous evidence to check first.

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