feat(hyperhdr): vendor the flatbuffers schemas and generated bindings

The .fbs files come from HyperHDR (see schema/NOTICE); the *_generated.rs
files are flatc output regenerated by scripts/regen-fbs.sh, which refuses to
run unless flatc matches the pinned flatbuffers crate.

scripts/regen-fbs.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Regenerate Rust code from the HyperHDR flatbuffers schemas.
+# The flatc version must match the `flatbuffers` crate version in Cargo.toml.
+set -euo pipefail
+
+root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+out="$root/src/hyperhdr/generated"
+
+crate_ver="$(grep -E '^flatbuffers\s*=' "$root/Cargo.toml" | grep -oE '[0-9]+\.[0-9]+\.[0-9]+')"
+flatc_ver="$(flatc --version | grep -oE '[0-9]+\.[0-9]+\.[0-9]+')"
+if [[ "$crate_ver" != "$flatc_ver" ]]; then
+    echo "flatc $flatc_ver does not match flatbuffers crate $crate_ver" >&2
+    exit 1
+fi
+
+rm -f "$out"/*_generated.rs
+mkdir -p "$out"
+flatc --rust -o "$out" "$root/schema/hyperhdr_request.fbs" "$root/schema/hyperhdr_reply.fbs"
+echo "generated with flatc $flatc_ver:"
+ls -1 "$out"