chore(ci): run the nightly-only checks again and raise the serde floor

[thatssoheil]

Fixes #851.

ci/test-stable.sh gated its benchmark check and its minimal-versions check on
RUST_VERSION, and no workflow sets that variable - ci.yml sets RUSTFLAGS,
RUST_BACKTRACE and nightly, none of them RUST_VERSION. So the block at
ci/test-stable.sh:14 has never been entered since #452, in the nightly job
included.

This gates the block on the toolchain actually in use instead. The nightly job
runs rustup update $nightly && rustup default $nightly before invoking the
script (ci.yml), so rustc --version reports a nightly version there and a
stable version in the stable matrix. Nothing else changes for the stable
jobs.

Enabling the gate exposes the second half of the issue: the check fails on the
serde floor the manifest declares. -Z minimal-versions resolves serde
1.0.60, whose alloc feature does not compile under it:

error[E0432]: unresolved imports `alloc::BTreeMap`, `alloc::BTreeSet`,
`alloc::BinaryHeap`, `alloc::LinkedList`, `alloc::VecDeque`
   --> serde-1.0.60/src/lib.rs:192:21

So the floor moves to 1.0.70. That number is measured, not guessed - see below.

Both changes have to land together: enabling the gate alone turns the nightly
job red, and publish_docs needs nightly, so documentation would stop
publishing too.

Evidence

The gate is dead. Simulating the CI jobs with rustc/cargo shims that log
every invocation, ci/test-stable.sh as it stands on master makes exactly two
cargo calls under a nightly toolchain and never reaches the block; with this
change it makes six, including cargo check --benches,
cargo hack --remove-dev-deps --workspace, cargo update -Z minimal-versions
and cargo check --all-features. Under a stable toolchain it still makes two.

1.0.70 is the first working floor. In a scratch clone, with
cargo hack --remove-dev-deps --workspace emulated (the crate does not have
cargo-hack installed here) followed by cargo +nightly update -Z minimal-versions:

serde floor	resolved	cargo +nightly check --all-features
1.0.60	1.0.60	rc=101, E0432
1.0.69	1.0.69	rc=101, E0432
1.0.70	1.0.70	rc=0

cargo +nightly check --benches is rc=0 with the floor at 1.0.70.

Stable is unaffected. cargo test (248 passed), cargo test --all-features
and cargo test --no-default-features --features serde all pass on the change.
The manifest's rust-version is untouched, so the minrust job is unaffected.

One trade-off worth naming

If rustc were somehow unresolvable the condition would read as false and the
block would skip silently. That is the same shape of failure that produced this
issue, so it is worth a maintainer's eye; I kept it because by that point in the
script cargo has already run successfully under set -e, and because this is
the form the issue's own comment proposes. Happy to switch to
rustup show active-toolchain or an explicit command -v rustc guard if you
would rather not rely on the version banner.

Disclosure

AI-assisted (Hermes Agent, Nous Research). Every claim above was re-verified
against the real toolchains before opening this; I can walk through any line.

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Pull request overview

Updates nightly CI detection and raises the Serde minimum version so minimal-version checks pass.

Changes:

• Detect nightly via rustc --version.
• Raise the Serde floor from 1.0.60 to 1.0.70.

File summaries

File	Description
ci/test-stable.sh	Re-enables nightly benchmark and minimal-version checks.
Cargo.toml	Updates the Serde dependency floor.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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