build: lift MSRV to 1.80 workspace / 1.88 node binding

The previous MSRV pins were below what current transitive dependencies
need:

  - rayon-core 1.13.0 (pulled in by the optional `parallel` feature on
    wickra-core via rayon) requires rustc >= 1.80; under the old 1.75
    workspace MSRV the CI MSRV job broke with "package `rayon-core
    v1.13.0` cannot be built because it requires rustc 1.80 or newer".
  - napi-build 2.3.2 (the build-script crate that napi-derive 2.x calls
    into) requires rustc >= 1.88; under the old 1.77 node-binding MSRV
    the CI MSRV-node job broke with "package `napi-build v2.3.2` cannot
    be built because it requires rustc 1.88 or newer".

Pinning the deps backwards would have frozen us out of upstream
security/fix releases for both crates. Lifting the MSRV is the cleaner
path for a young 0.x library — downstream consumers on older toolchains
can stay on the already-published 0.2.0.

Updated:
  - Cargo.toml workspace rust-version 1.75 -> 1.80
  - bindings/node/Cargo.toml rust-version 1.77 -> 1.88
  - .github/workflows/ci.yml MSRV matrix names + toolchain values + comment
  - CHANGELOG.md [Unreleased] documents the bump

CHANGELOG.md

@@ -7,6 +7,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- **MSRV bumped.** Workspace minimum supported Rust version is now **1.80**
+  (was 1.75) and the Node binding (`wickra-node`) is now **1.88** (was 1.77).
+  The bumps are driven by transitive-dependency floors that were lifted in
+  recent updates: `rayon-core >= 1.13.0` requires Rust 1.80, and
+  `napi-build >= 2.3.2` requires Rust 1.88. Pinning the deps to the older
+  versions would have frozen us out of future security fixes from those
+  upstreams, so lifting the MSRV is the cleaner path for a young 0.x
+  library. Downstream consumers on older Rust toolchains can stay on
+  Wickra 0.2.0.
+
 ## [0.2.0] - 2026-05-23
 
 ### Fixed