Add CI job to verify fuzz crate builds

Add a Fuzz job to the CI workflow that builds the fuzz crate on pull
requests. This catches compilation errors without spending CI cycles
on actual fuzzing (which provides minimal value in short PR runs).

For reference, rust-bitcoin uses a similar approach: PRs only build,
while a separate cron-daily-fuzz.yml runs targets for ~1 hour each.

Closes #1313

Author: DanGould

.github/workflows/rust.yml

@@ -144,3 +144,16 @@ jobs:
         with:
           name: mutants-incremental-cargo.out
           path: mutants.out
+
+  Fuzz:
+    name: Fuzz build
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@v4
+      - name: "Install nightly toolchain"
+        uses: dtolnay/rust-toolchain@nightly
+      - name: "Use cache"
+        uses: Swatinem/rust-cache@v2
+      - name: "Build fuzz targets"
+        run: cd fuzz && cargo build