fix(ci): update MSRV to 1.88, fix Rust 1.92 clippy lints

[aWN4Y25pa2EK]

Summary

Fix CI failures after crates.io publish.

Changes

• CI: Update MSRV job from 1.82 to 1.88 (matching Cargo.toml)
• Clippy: Fix manual_is_multiple_of lint in keyring.rs
• Clippy: Fix manual_midpoint lint in m2m_ai_test.rs
• README: Update Quick Start with correct crates.io install instructions

README Updates

# Cargo.toml
[dependencies]
m2m-protocol = "0.4"

# With cryptographic security
m2m-protocol = { version = "0.4", features = ["crypto"] }

Verification

• cargo clippy --all-features -- -D warnings passes
• All tests pass