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

Cargo.toml

@@ -15,7 +15,7 @@ exclude = ["fuzz"]
 version = "0.2.0"
 authors = ["kingchenc <kingchencp@gmail.com>"]
 edition = "2021"
-rust-version = "1.75"
+rust-version = "1.80"
 license = "PolyForm-Noncommercial-1.0.0"
 repository = "https://github.com/kingchenc/wickra"
 homepage = "https://github.com/kingchenc/wickra"