rust/msrv: 1.85.0 → 1.88.0

`cargo check` fails because `human-panic` and `sysinfo` require at least
1.88.0.

    +++ command cargo check --release --locked --all-targets
    error: rustc 1.85.0 is not supported by the following packages:
      human-panic@2.0.6 requires rustc 1.88
      sysinfo@0.37.2 requires rustc 1.88

Bump MSRV to fix this.

1.88.0 introduced [let chains], which in turn has `clippy` warn about
nested if statements. All of these sites are fixed in this change.

1.87.0 introduced [`is_multiple_of`], which is a more readable version
of `x % y == 0`.

[let chains]: https://blog.rust-lang.org/2025/06/26/Rust-1.88.0/#let-chains
[`is_multiple_of`]: https://doc.rust-lang.org/std/primitive.usize.html#method.is_multiple_of

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## Minimum Supported Rust Version (MSRV)
+
+The MSRV is updated to `1.88.0` due to dependencies requiring a higher bound.
+This effectively puts all published crates in `heartwood` to an MSRV of `1.88.0`.
+
 ## Domain Name Migration
 
 Following a domain move of the project, the names of the bootstrap nodes change: