feat(wfe-buildkit-protos): generate full BuildKit gRPC API (tonic 0.14)

New crate generating Rust gRPC stubs from the official BuildKit
proto files (git submodule from moby/buildkit). Control service,
LLB definitions, session protocols, and source policy.
tonic 0.14 / prost 0.14.

wfe-buildkit-protos/build.rs

@@ -0,0 +1,58 @@
+use std::path::PathBuf;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let buildkit_root = PathBuf::from("vendor/buildkit");
+
+    // Use Go-style import paths so protoc sees each file only once
+    let proto_dir = PathBuf::from("proto");
+    let go_prefix = "github.com/moby/buildkit";
+
+    let proto_files: Vec<PathBuf> = vec![
+        // Core control service (Solve, Status, ListWorkers, etc.)
+        "api/services/control/control.proto",
+        // Types
+        "api/types/worker.proto",
+        // Solver / LLB definitions
+        "solver/pb/ops.proto",
+        // Source policy
+        "sourcepolicy/pb/policy.proto",
+        // Session protocols
+        "session/auth/auth.proto",
+        "session/filesync/filesync.proto",
+        "session/secrets/secrets.proto",
+        "session/sshforward/ssh.proto",
+        "session/upload/upload.proto",
+        "session/exporter/exporter.proto",
+        // Utilities
+        "util/apicaps/pb/caps.proto",
+        "util/stack/stack.proto",
+    ]
+    .into_iter()
+    .map(|p| proto_dir.join(go_prefix).join(p))
+    .collect();
+
+    println!(
+        "cargo:warning=Compiling {} buildkit proto files",
+        proto_files.len()
+    );
+
+    let mut prost_config = prost_build::Config::new();
+    prost_config.include_file("mod.rs");
+
+    tonic_prost_build::configure()
+        .build_server(false)
+        .compile_with_config(
+            prost_config,
+            &proto_files,
+            // Include paths for import resolution:
+            // 1. The vendor dir inside buildkit (for Go-style github.com/... imports)
+            // 2. The buildkit root itself (for relative imports)
+            // 3. Our proto/ dir (for google/rpc/status.proto)
+            &[
+                // proto/ has symlinks that resolve Go-style github.com/... imports
+                PathBuf::from("proto"),
+            ],
+        )?;
+
+    Ok(())
+}