Declare psm MSRV

[tisonkun]

Declare Rust 1.88 as the minimum supported Rust version for psm, matching ar_archive_writer 0.5.2, and add dedicated MSRV CI coverage.

Addresses #139.

[nagisa]

nack, I don't think downgrading the dep is on the table; we also don't really need to maintain msrv that low at this point. annotating rust-version for psm appropriately seems like the most reasonable thing.

[tisonkun]

CI failure doesn't seem related but downloading failures.

[tisonkun]

@nagisa That's OK. But perhaps we should still define an MSRV for psm? In this case, ar_archive_writer requires 1.88 which we may use. Or use an even-earilier ar_archive_writer to keep lower MSRV.

I personally prefer higher MSRV because old Rust versions are anyway no maintained. Even I'd prefer the Rust team says that old versions are unmaintained so bumping your MSRV is preferred :D

[nagisa]

Yeah, as I said I would be okay with specifying the factual rust-version for psm. Not that it would help your referenced use-case in stacksafe because crates.io already has releases that don't specify rust-version.

[tisonkun]

@nagisa OK. I've update the MSRV of psm to 1.88.

For stacksafe case, I think I can update its MSRV to 1.88 as well and at least later we should not face this rust-version incompatible issue.