## What
Updates the optional `cargo-nextest` install command in
`docs/install.md`:
- `cargo install cargo-nextest` -> `cargo install --locked
cargo-nextest`
## Why
The current docs command can fail during source install because recent
`cargo-nextest` releases intentionally require `--locked`.
Repro (macOS, but likely not platform-specific):
- `cargo install cargo-nextest`
- Fails with a compile error from `locked-tripwire` indicating:
- `Nextest does not support being installed without --locked`
- suggests `cargo install --locked cargo-nextest`
Using the locked command succeeds:
- `cargo install --locked cargo-nextest`
## How
Single-line docs change in `docs/install.md` to match current
`cargo-nextest` install requirements.
## Validation
- Reproduced failure locally using a temporary `CARGO_HOME` directory
(clean Cargo home)
- Example command used: `CARGO_HOME=/tmp/cargo-home-test cargo install
cargo-nextest`
- Confirmed success with `cargo install --locked cargo-nextest`
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