build: standardize Rust toolchain on 1.93 (PPA build, MSRV, CI verification)

[inureyes]

Summary

Standardize the project's Rust toolchain handling on 1.93, across three areas:

1. Launchpad PPA build toolchain → rustc-1.93 (1.93.1). The ~lablup/+archive/ubuntu/rustc-release PPA now publishes rustc-1.93 (1.93.1) for jammy and noble, and Ubuntu 26.04 (resolute) ships 1.93.1 in its archive — so jammy/noble no longer need to stay on rustc-1.92.
2. Workspace MSRV → 1.93. The previously declared MSRV (1.88) was inaccurate (see below).
3. New CI msrv job that verifies the declared MSRV so it can no longer silently drift.

Why the MSRV bump (the important part)

rustyline 18 (a direct dependency) calls File::lock/File::lock_shared — the file_lock feature, stabilized in Rust 1.89. So the workspace has not compiled on 1.88 since rustyline was adopted, even though we declared rust-version = "1.88".

Verified locally with cargo check --workspace --locked:

Toolchain	Result
1.88.0	❌ error[E0658]: use of unstable library feature 'file_lock'
1.89.0	✅
1.93.0	✅

CI never caught this because it only builds on stable. The real hard floor is 1.89; the declared MSRV is set to 1.93 to align with the Launchpad PPA build toolchain.

Changes

PPA toolchain (39ff5b5f)

• debian/control{,.source}: Build-Depends rustc-1.93 | rustc (>= 1.93), cargo-1.93 | cargo (>= 1.93)
• debian/rules{,.source,.launchpad,.launchpad-simple}: prefer rustc-1.93/cargo-1.93, require >= 1.93
• .github/workflows/launchpad_ppa.yml: vendor_rust 1.92.0 → 1.93.1 (all distros)
• debian/README.packaging: version references

MSRV + CI (255dc630)

• Cargo.toml, crates/bssh-russh/Cargo.toml: rust-version 1.88 → 1.93
• .github/workflows/ci.yml: new msrv job — reads rust-version from Cargo.toml (so the job never drifts from the declaration) and runs cargo check --workspace --locked on that toolchain. The main ci job stays on stable.

Distribution behavior (all converge on 1.93.1)

Distro	rustc source
jammy	PPA rustc-1.93 (1.93.1)
noble	PPA rustc-1.93 (1.93.1)
resolute	archive rustc 1.93.1 (rust-defaults)

Notes

• No rust-toolchain.toml was added intentionally: pinning would force all rustup builds (local/CI/release) onto one version and drop stable coverage, and it wouldn't affect the Launchpad build anyway (that uses the system cargo-1.93, not rustup).
• crates/bssh-russh-sftp does not declare a rust-version; left out of scope here but could be set to 1.93 for consistency since it is published.

Test plan

• cargo +1.88.0 check --workspace --locked → fails (E0658), confirming the floor
• cargo +1.89.0 check --workspace --locked → passes
• cargo +1.93.0 check --workspace --locked → passes
• ci.yml / launchpad_ppa.yml YAML validated; MSRV extraction yields 1.93
• CI msrv job green on this PR