P1: AsyncVfsBackend trait design (Phase 1 - framework)

- Add AsyncVfsBackend + AsyncVfsFile trait definitions
- Use cfg(feature = "async-vfs") for optional compilation
- Design notes for Phase 2-5 implementation
- Estimated: ~13 hours (multi-day project)

Phase 2: AsyncLocalFs (tokio::fs)
Phase 3: AsyncS3Vfs (async client)
Phase 4: AsyncSmbVfs (async wrapper)
Phase 5: WebDAV integration

Tests: 289 passed, 0 failed

markbase-core/Cargo.toml

@@ -85,6 +85,7 @@ tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 default = []                          # 默认不启用可选格式
 optional-formats = ["unrar", "xz2", "sevenz-rust"]  # 争议格式可选启用
 smb-server = ["dep:smb-server"]       # SMB server feature flag
+async-vfs = []                        # Async VfsBackend trait (Phase 1 - design only)
 
 [dev-dependencies]
 # tempfile moved to dependencies (needed for archive extraction)