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/.claude/rules/docs-maintenance.md

@@ -0,0 +1,12 @@
+When modifying code in a directory that contains a `CLAUDE.md` file, check whether your changes affect the documented
+architecture, key decisions, or gotchas. If they do, update the `CLAUDE.md` to stay in sync. If you notice a `CLAUDE.md`
+missing in a directory where there should be one, add it. Skip this for trivial changes (bug fixes, formatting, small
+refactors that don't change the architecture).
+
+If something failed due to a wrong assumption, add a `Gotcha/Why` entry to the nearest `CLAUDE.md`.
+
+Add `Decision/Why` entries to the nearest colocated `CLAUDE.md` for key decisions. If the decision has rich evidence
+(benchmarks, detailed analysis), put the evidence in `docs/notes/` and link from the CLAUDE.md.
+
+When writing guides, see [this diff](https://github.com/vdavid/cmdr/commit/13ad8f3#diff-795210f) for the formatting
+standard. (Before: AI-written. After: matching our standards for conciseness and clarity.)