feat(wfe-buildkit, wfe-containerd): add container executor crates

Standalone workspace crates for BuildKit image building and containerd
container execution. Config types, YAML schema integration, compiler
dispatch, validation rules, and mock-based unit tests.

Current implementation shells out to buildctl/nerdctl — will be
replaced with proper gRPC clients (buildkit-client, containerd protos)
in a follow-up. Config types, YAML integration, and test infrastructure
are stable and reusable.

wfe-buildkit: 60 tests, 97.9% library coverage
wfe-containerd: 61 tests, 97.8% library coverage
447 total workspace tests.

wfe-containerd/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "wfe-containerd"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+homepage.workspace = true
+description = "containerd container runner executor for WFE"
+
+[dependencies]
+wfe-core = { workspace = true }
+tokio = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+async-trait = { workspace = true }
+tracing = { workspace = true }
+thiserror = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tokio = { workspace = true, features = ["test-util"] }
+tempfile = { workspace = true }
+tokio-util = "0.7"