noq-proto: fix CID exhaustion check overflow on 32-bit targets

[KevinRoebert]

Summary

Fixes an overflow in Endpoint::cids_exhausted() on 32-bit targets when CID length is 4.

The previous check used usize exponentiation:

• 2usize.pow(cid_len * 8) overflows for cid_len == 4 on 32-bit platforms (2^32 does not fit in usize).

This change is already merged into quinn: quinn-rs/quinn#2533

Changes

• Replaced the exhaustion arithmetic with an overflow-safe implementation using u64.

Notes

• Kept the same exhaustion threshold semantics (>3/4 used).
• Preserved the early return behavior for cid_len == 0 and cid_len > 4.
• No behavior change is intended on 64-bit targets.
• Arithmetic is now architecture-independent.

[flub]

Thanks!

We do need to merge Quinn's changes into noq again sometime soon. And really need to set up a more structured approach to that.