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/worker/Cargo.toml

@@ -17,6 +17,7 @@ adapters-nats      = { workspace = true }
 adapters-event-transport = { workspace = true }
 adapters-exif      = { workspace = true }
 adapters-thumbnail = { workspace = true }
+adapters-sidecar   = { workspace = true }
 async-nats         = { workspace = true }
 
 futures           = { workspace = true }