fix: the published MSRV was false, and CI now verifies it

[ccross2]

The claim was not stale — it was false

Cargo.toml declared rust-version = "1.75". Measured against the resolved dependency graph:

346 packages, 251 declaring a rust-version
 59 require > 1.75, including DIRECT dependencies:
      ort    1.88
      image  1.88
      zbus   1.87

The true floor is 1.88. The crate could not build on the version it advertised, and anyone who trusted the field got a failure deep inside a dependency rather than a clear "your toolchain is too old". Nothing checked it.

This is the fifth instance in this repository of a claim in one artifact that nothing in another artifact enforces — after the release trigger (#75), the network-isolation claim (#78), and TimeoutStopSec.

A new msrv job reads the version from Cargo.toml, so the manifest stays the single source of truth and the job cannot drift from it. It refuses rather than proceeding if the field is absent — the same failure shape as the release trigger, so it gets the same treatment.

Correction to #87

That PR avoided clippy's suggested as_chunks::<4>() on the grounds that it is stable since 1.88 and would "raise MSRV by thirteen minor versions".

ort already required 1.88. The constraint I was honouring did not exist.

The code is still correct — indexing works on every toolchain and couples to no lint name — but the stated justification overstated the cost. Recorded here rather than quietly dropped, because that reasoning sits in a merged commit message, a PR body and an ADR.

Two CI drift fixes of the same class

• cargo install cargo-deb pinned to 3.7.0. It produces the shipped artifact, and an unpinned install is exactly the drift that took CI down for six days on 2026-08-24. It happened to resolve to the same version locally and in CI that day — by luck, not design.
• Both clippy invocations gain --all-targets. The five integration tests added in test: the first integration tests, and a live defect the first one found #91 were clean locally and not linted in CI at all.

Verification

• actionlint exit 0.
• The MSRV extraction returns 1.88; a manifest with no rust-version trips the refusal (negative control).
• fmt clean, clippy --workspace --all-targets -D warnings clean, 93 tests pass, 0 failures.

Not verified locally: that the workspace builds at 1.88. This machine has 1.95 and no 1.88 available. cargo check --workspace --all-targets --locked passes at 1.95, which is a proxy only — the new msrv job is the real check, and this PR is its first run. If it fails, the floor is higher than the metadata declares and the value moves again.

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