Persist fuzz corpus between CI runs

Implements a persistent, global fuzz corpus cache.

PRs perform a "read-only" restore from the `main` cache
to seed fuzzer runs. The `main` branch performs a
"read-write" to save new findings and grow the corpus.

Author: Anyitechs

.github/workflows/build.yml

@@ -267,6 +267,29 @@ jobs:
       - name: Install Rust ${{ env.TOOLCHAIN }} toolchain
         run: |
           curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --profile=minimal --default-toolchain ${{ env.TOOLCHAIN }}
+      # This is read-only for PRs. It seeds the fuzzer for a more effective run.
+      # NOTE: The `key` is unique and will always miss, forcing a fallback to
+      # the `restore-keys` to find the latest global cache from the `main` branch.
+      - name: Restore persistent fuzz corpus (PR)
+        if: ${{ github.ref != 'refs/heads/main' }}
+        uses: actions/cache/restore@v4
+        with:
+          path: fuzz/hfuzz_workspace
+          key: fuzz-corpus-${{ github.ref }}-${{ github.sha }}
+          restore-keys: |
+            fuzz-corpus-refs/heads/main-
+      # The `restore-keys` performs a prefix search to find the most recent
+      # cache from a previous `main` run. We then save with a new, unique
+      # `key` (using the SHA) to ensure the cache is always updated,
+      # as caches are immutable.
+      - name: Restore/Save persistent honggfuzz corpus (Main)
+        if: ${{ github.ref == 'refs/heads/main' }}
+        uses: actions/cache@v4
+        with:
+          path: fuzz/hfuzz_workspace
+          key: fuzz-corpus-refs/heads/main-${{ github.sha }}
+          restore-keys: |
+            fuzz-corpus-refs/heads/main-
       - name: Sanity check fuzz targets on Rust ${{ env.TOOLCHAIN }}
         run: |
           cd fuzz