Correct the minimum Rust version

[onatozmenn]

What this changes

Raises the declared minimum Rust version from 1.85 to 1.88, updates the installation text and changelog, and adds a CI job that checks the workspace with Rust 1.88.0.

It also applies the mechanical Clippy cleanups that become active once the declared MSRV includes let chains and is_multiple_of.

Why this way

The released v0.2.11 source uses let chains in the parser and resolver. A real build with Rust 1.87 fails at those sites with E0658, while Rust 1.88 builds the workspace and runs the MCP server. Keeping an exact 1.88 CI job makes the metadata an enforced boundary instead of an untested claim.

What it still does not do

This does not change the Deed language or generated programs. It corrects the Rust toolchain required to build the implementation.

Checks

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• 2,092 targeted nextest tests passed, 4 existing skips
• cargo check --workspace --locked in a Rust 1.88.0 Linux container
• Rust 1.87.0 now refuses the workspace from rust-version instead of reaching E0658
• uvx --from actionlint-py actionlint .github/workflows/ci.yml
• A deliberately invalid workflow copy was rejected by actionlint
• git diff --check
• Full workspace tests (running in CI)
• Compiler corpus checks (no compiler behavior changed)
• Design document update (not applicable)
• Regression gate added for the minimum Rust version