SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

vendor/smb2/src/testing/CLAUDE.md

@@ -0,0 +1,48 @@
+# Testing module -- Docker-based SMB test servers
+
+Feature-gated (`testing` feature flag). Provides Docker-based Samba containers for consumers (apps that depend on smb2) to test their SMB integration.
+
+## Key files
+
+| File | Purpose |
+|---|---|
+| `mod.rs` | `TestServers`, `Error`, port constants, embedded Docker files, `write_compose_files()` |
+
+## Architecture
+
+Three-layer testing model:
+
+1. **Layer 1 (Rust)**: `TestServers::start()` / `start_all()` / `start_blocking()` return a struct with `*_client()` methods that connect to Docker containers.
+2. **Layer 2 (E2E)**: `write_compose_files(dir)` extracts embedded Docker infrastructure to disk for non-Rust test frameworks.
+3. **Layer 3 (Manual QA)**: Same compose files, run manually.
+
+## Embedded files
+
+All 35 Docker files (compose, Dockerfiles, smb.conf, scripts) are embedded via `include_str!` at compile time. At runtime, `write_compose_files()` writes them to a temp directory. Docker Compose runs from there.
+
+## Port scheme
+
+15 containers on ports 10480-10494. Each port has an env-var override (`SMB_CONSUMER_*_PORT`). The `port()` function checks the env var, falls back to the hardcoded default.
+
+## Profiles
+
+- **Minimal**: guest + auth only (2 containers, fast startup).
+- **All**: all 15 containers.
+
+Calling a `*_client()` method for a container not in the current profile returns `Error::ContainerNotStarted`.
+
+## Key decisions
+
+| Decision | Choice | Why |
+|---|---|---|
+| No extra deps | `std::process::Command` for Docker | Keep the crate lean |
+| Temp dir via `std::env::temp_dir()` | No `tempfile` crate | No extra deps |
+| Embedded files via `include_str!` | Self-contained published crate | Consumers don't need smb2 source tree |
+| Separate error type | `testing::Error` vs `smb2::Error` | Docker failures are not protocol errors |
+| Best-effort cleanup in Drop | `docker compose down` | LazyLock statics never drop, so this is convenience only |
+
+## Gotchas
+
+- **LazyLock statics never drop**: `TestServers::drop()` won't run at process exit. CI should use explicit cleanup steps.
+- **Flaky container has no health check**: The 5s-up/5s-down cycle means health checks would randomly fail. `wait_healthy()` skips it.
+- **DFS is disabled on test clients**: Consumer containers don't set up DFS. The `connect_guest` / `connect_auth` helpers set `dfs_enabled: false`.