Cargo.lock handling to be same across examples

[finalyards]

I'm... having second thoughts about this, but since I volunteered in #482 to make one, here goes.

We can continue discussion here - the PR is intended to fulfil the specs in that discussion.

[finalyards]

I made a second branch, last week, the diff to origin/main is there.

Disclaimer: I'm not claiming this to be the truth. I'm just trying to make the Cargo.lock policy least invasive for me, as a developer, and keep it useful for the repo. I first thought my approach is disruptive, but Copilot sees it more as a continuation of the stated Cargo guidelines. (granted, Copilot has a tendency to agree; you be the judge of it)

In short, I would:

• remove Cargo.lock's from the examples/
• retain them for tests
• ..which involved adding one for host/
• ..with the assumption (mildly googled) that it will not be used by Cargo, when using host as a dependency. It's meant for the tests

This move keeps one or two Cargo.locks so that CI caching can be based on them, and cargo test --locked would keep some consistency to the runs.

Let me know what you think. If you show 👍 for the approach, I'll make the PR represent it, instead.

ps. I asked Copilot to summarize the approach as the CONTRIBUTING.md.

[lulf]

Lgtm