feat: real XMP sidecar adapter, replaces LogSidecarWriter stubs

- adapters-sidecar: XmpSidecarWriter using xmp_toolkit
- Writes StructuredData → XMP with EXIF/DC/XMP namespace routing
- Reads XMP back to StructuredData
- Wired into bootstrap + worker, deleted both LogSidecarWriter stubs

crates/adapters/sidecar/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "adapters-sidecar"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+domain       = { workspace = true }
+async-trait  = { workspace = true }
+tokio        = { workspace = true, features = ["fs"] }
+xmp_toolkit  = "1.11"
+tracing      = { workspace = true }