TASK TOOLPIN-E-4021 — Pin the Rust toolchain to 1.97.0

[olympum]

build: pin the Rust toolchain to 1.97.0

This repository was invisible to the TOOLPIN plan, which enumerated nine Rust
repositories from a hand-derived list. scripts/ci/rust_pin_audit.py derives them
from their trees instead, and found this one on its first live run.

1.97.0 is the ratified estate version (D207).

One target, measured rather than guessed:

aarch64-apple-darwin    .github/workflows/component-artifacts.yml

Verified under the pinned compiler:

rustc in-tree   rustc 1.97.0 (2d8144b78 2026-07-07)
gate            cargo clippy --workspace --all-targets -- -D warnings exit 0; cargo test --workspace exit 0, 100 + 4 tests

src/main.rs carries three clippy-1.97 changes, and they are not all the same kind.

Two are fixes. clippy::ptr_arg fires on the unmodified source under 1.97.0, so
Option<&PathBuf> becomes Option<&Path> and as_ref() becomes as_deref().

One is a SUPPRESSION, stated as such: #[allow(clippy::too_many_arguments)] on
onboard_output. The function took eight arguments before this change and takes
eight after, so its arity did not move -- 1.97.0 tightened the default. The
suppression is defensible for a print helper whose arguments are all distinct
values, but it is a silenced lint rather than a fixed one, and it should not be
absorbed into 'pinned the toolchain'.

Both lints were confirmed to fire by reverting the source, keeping the pin, and
running the repository's own clippy invocation:

clippy::ptr_arg
clippy::too_many_arguments

Fixes #4021