fix(ci): add Cross.toml to install protoc in cross Docker containers

After the cargo clean fix resolved the GLIBC mismatch, cross builds now
compile from scratch inside Docker containers which lack protobuf-compiler.
Add Cross.toml with pre-build hooks for aarch64-unknown-linux-gnu and
x86_64-unknown-linux-musl targets to install protoc before building.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: doublegate

Cross.toml

@@ -0,0 +1,11 @@
+# Cross-compilation configuration for cross-rs
+# See: https://github.com/cross-rs/cross/wiki/Configuration
+#
+# Installs protobuf-compiler inside Docker containers so prost-build
+# can compile .proto files during cross-compilation builds.
+
+[target.aarch64-unknown-linux-gnu]
+pre-build = ["apt-get update && apt-get install -y protobuf-compiler"]
+
+[target.x86_64-unknown-linux-musl]
+pre-build = ["apt-get update && apt-get install -y protobuf-compiler"]