Include cxxbridge-cmd in Cargo.lock, check version consistency (#3712)

This adds cxxbridge-cmd to Cargo.lock per
https://github.com/dtolnay/cxx/issues/1407#issuecomment-2509136343

It adds an MSRV to `src/taskchampion-cpp/Cargo.toml` so that the
version of `Cargo.lock` is stil compatible with the MSRV.

It additionally adds a check of the Cargo metadata for all of the cxx*
versions agreeing, and for the MSRV's agreeing.

.github/workflows/checks.yml

@@ -64,3 +64,19 @@ jobs:
         with:
           command: fmt
           args: --all -- --check
+
+  cargo-metadata:
+    runs-on: ubuntu-latest
+    name: "Cargo Metadata"
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          components: rustfmt
+          toolchain: stable
+          override: true
+
+      - name: "Check metadata"
+        run: ".github/workflows/metadata-check.sh"