Fix stable build on Ubuntu 26.04

[erpalma]

Summary

• avoid the unstable if let match guard in the core poll loop so the project builds on stable Rust
• remove a platform-specific redundant shm_segsz as usize cast that fails the CI clippy gate on Ubuntu 26.04

Validation

• cargo +nightly fmt
• cargo check --bin yserver
• cargo clippy --all-targets -- -D warnings

[joske]

@erpalma if-let guard IS stable since 1.95, but that is quite recent, and may unblock other distros.

That said, the workflow wants signed/verified commits, so please sign your commit with git commit --amend --no-edit -S