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

.github/workflows/ci.yml

@@ -56,9 +56,12 @@ jobs:
         run: cargo build -p wickra-examples --bins
 
   # Verify the crates still build and test on their declared minimum supported
-  # Rust version. The workspace pins rust-version = "1.75"; bindings/node needs
-  # 1.77 because napi-build emits `cargo::` directives. Without this job an
-  # accidental use of a newer API would only surface for downstream users.
+  # Rust version. The workspace pins rust-version = "1.80" (raised from 1.75
+  # so rayon-core >= 1.13.0 keeps compiling); bindings/node pins
+  # rust-version = "1.88" because napi-build 2.3.2 requires it (and that
+  # subsumes the older 1.77 floor needed for `cargo::` directives). Without
+  # this job an accidental use of a newer API would only surface for
+  # downstream users.
   msrv:
     name: ${{ matrix.name }}
     runs-on: ubuntu-latest
@@ -66,11 +69,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - name: MSRV workspace (Rust 1.75)
-            toolchain: "1.75"
+          - name: MSRV workspace (Rust 1.80)
+            toolchain: "1.80"
             packages: "-p wickra-core -p wickra -p wickra-data"
-          - name: MSRV node binding (Rust 1.77)
-            toolchain: "1.77"
+          - name: MSRV node binding (Rust 1.88)
+            toolchain: "1.88"
             packages: "-p wickra-node"
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1