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/pack/CLAUDE.md

@@ -0,0 +1,45 @@
+# Pack -- binary serialization primitives
+
+Cursor-based binary reader/writer for SMB2 wire format. Hand-rolled, no proc macros.
+
+## Key files
+
+| File | Purpose |
+|---|---|
+| `mod.rs` | `ReadCursor`, `WriteCursor`, `Pack`/`Unpack` traits, primitive read/write methods |
+| `guid.rs` | GUID pack/unpack with mixed-endian layout |
+| `filetime.rs` | Windows FILETIME (100ns ticks since 1601-01-01) to/from `SystemTime` |
+
+## Core types
+
+- **`ReadCursor<'a>`**: Reads from `&[u8]` with position tracking. Returns `Error` on buffer overrun (no panics). All reads are little-endian.
+- **`WriteCursor`**: Writes into a growable `Vec<u8>`. Supports backpatching (`set_u16_le_at`, `set_u32_le_at`) for length fields written before their values are known. `align_to(n)` pads with zeros to n-byte boundary.
+- **`Pack` trait**: `fn pack(&self, cursor: &mut WriteCursor)` -- serialize to binary.
+- **`Unpack` trait**: `fn unpack(cursor: &mut ReadCursor) -> Result<Self>` -- deserialize from binary.
+
+## GUID mixed-endian layout
+
+Windows GUIDs have a mixed-endian wire format:
+- `data1` (u32): little-endian
+- `data2` (u16): little-endian
+- `data3` (u16): little-endian
+- `data4` ([u8; 8]): raw bytes (no endian conversion)
+
+This matches the COM/DCOM convention. Not the same as RFC 4122 UUID byte order.
+
+## FileTime conversion
+
+Windows FILETIME: 100-nanosecond intervals since 1601-01-01 00:00:00 UTC.
+Unix epoch: 1970-01-01 00:00:00 UTC.
+Offset: 11,644,473,600 seconds (116,444,736,000,000,000 ticks).
+
+## Key decisions
+
+- **Hand-rolled instead of proc macros**: Full control over wire format details (offsets, alignment, backpatching). Easier to debug. No build-time dependency.
+- **`MAX_UNPACK_BUFFER` (16 MB)**: `read_bytes_bounded` refuses allocations larger than 16 MB. Prevents OOM from malicious packets claiming huge lengths.
+
+## Gotchas
+
+- **Everything is little-endian**: Except TCP framing (see transport module). ReadCursor/WriteCursor only do LE.
+- **UTF-16LE byte length must be even**: `read_utf16_le` returns an error on odd byte counts.
+- **Backpatching requires placeholder**: Write a zero first, then `set_u32_le_at` to overwrite once the real value is known. Common pattern for length-prefixed fields.