Introduce CI action to enforce encodable fuzzing

diff --git a/.github/workflows/rust.yml b/.github/workflows/rust.yml
index 2337d2f0..263d648e 100644
--- a/.github/workflows/rust.yml
+++ b/.github/workflows/rust.yml
@@ -239,6 +239,21 @@ jobs:
       - name: "Check error type re-exports"
         run: contrib/check-error-reexports.sh
 
+  Encodable-coverage:
+    name: Check Encodable fuzz coverage - stable toolchain
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+        with:
+          persist-credentials: false
+      - name: "Select toolchain"
+        uses: dtolnay/rust-toolchain@5d458579430fc14a04a08a1e7d3694f545e91ce6 # stable
+      - name: "Check Encodable coverage"
+        run: ./contrib/check-encodable-coverage.sh
+
   DiffMutants:
     name: Check cargo mutants in diff - stable toolchain
     runs-on: ubuntu-24.04
diff --git a/contrib/check-encodable-coverage.sh b/contrib/check-encodable-coverage.sh
new file mode 100755
index 00000000..aafec533
--- /dev/null
+++ b/contrib/check-encodable-coverage.sh
@@ -0,0 +1,113 @@
+#!/usr/bin/env bash
+#
+# Check that all types implementing encoding::Encodable are covered in the fuzz
+# test that compares encoding between old and new bitcoin crates.
+
+set -euo pipefail
+
+REPO_DIR=$(git rev-parse --show-toplevel)
+FUZZ_FILE="$REPO_DIR/fuzz/fuzz_targets/bitcoin/compare_consensus_encoding.rs"
+TRAIT_IMPL_JS="$REPO_DIR/target/doc/trait.impl/bitcoin_consensus_encoding/encode/trait.Encodable.js"
+
+# Known exclusions (types that don't exist in old_bitcoin 0.32 or are generic).
+# Add types here that have new Encodable but no old_bitcoin equivalent.
+# - CommandString has very different decoding functionality in new bitcoin.
+# - HeadersMessage has no type in 0.32 bitcoin. Vec<(Header, u8)> is not Decodable.
+# - InventoryPayload has no type in 0.32 bitcoin. Vec<Inventory> fails special case consideration.
+# - FeeFilter is a FeeRate newtype. FeeRate has no old Encodable/Decodable and is just a u64 le encoding in FeeFilter.
+EXCLUSIONS="CommandString HeadersMessage InventoryPayload FeeFilter NetworkMessage Script Validation V2NetworkMessage V1MessageHeader"
+
+main() {
+    check_required_commands
+
+    generate_docs
+    check_trait_impl_file
+
+    local new_types fuzz_types missing
+    new_types=$(extract_new_types)
+    fuzz_types=$(extract_fuzz_types)
+    missing=$(find_missing_types "$new_types" "$fuzz_types")
+
+    if [ -n "$missing" ]; then
+        echo "The following types implement encoding::Encodable but are not in the fuzz test:" >&2
+        for type in $missing; do
+            echo "  - $type" >&2
+        done
+        err "Either add them to compare_consensus_encoding.rs or add to EXCLUSIONS in this script"
+    fi
+
+    echo "All encoding::Encodable types are covered (or excluded)"
+}
+
+generate_docs() {
+    echo "Generating docs to discover Encodable implementors..."
+    cargo doc --workspace --no-deps --quiet
+}
+
+check_trait_impl_file() {
+    if [ ! -f "$TRAIT_IMPL_JS" ]; then
+        err "Could not find trait implementors file at $TRAIT_IMPL_JS"
+    fi
+}
+
+# Extract type names from the rustdoc implementors JS file.
+# Split on '>' then find lines starting with TypeName< pattern, excluding "Encodable" itself.
+extract_new_types() {
+    tr '>' '\n' < "$TRAIT_IMPL_JS" \
+        | grep -oE '^[A-Z][a-zA-Z0-9_]+<' \
+        | sed 's/<$//' \
+        | grep -v '^Encodable$' \
+        | sort -u
+}
+
+# Extract types from the fuzz test file.
+extract_fuzz_types() {
+    grep -E 'compare_encoding!' "$FUZZ_FILE" \
+        | grep -v '//' \
+        | sed -E 's/.*compare_encoding!\s*\(\s*data\s*,\s*//' \
+        | sed -E 's/\s*\);.*//' \
+        | sed -E 's/,.*$//' \
+        | sed -E 's/.*:://' \
+        | grep -E '^[A-Z]' \
+        | sort -u
+}
+
+# Find types that are in new_types but not in fuzz_types or exclusions.
+find_missing_types() {
+    local new_types=$1
+    local fuzz_types=$2
+    local missing=""
+
+    for type in $new_types; do
+        if ! echo "$fuzz_types" | grep -qw "$type"; then
+            if ! echo "$EXCLUSIONS" | grep -qw "$type"; then
+                missing="$missing $type"
+            fi
+        fi
+    done
+
+    echo "$missing"
+}
+
+check_required_commands() {
+    need_cmd grep
+    need_cmd sed
+    need_cmd tr
+}
+
+err() {
+    echo "ERROR: $1" >&2
+    exit 1
+}
+
+need_cmd() {
+    if ! command -v "$1" > /dev/null 2>&1; then
+        err "need '$1' (command not found)"
+    fi
+}
+
+#
+# Main script
+#
+main "$@"
+exit 0