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

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "crates/adapters/nats",
     "crates/adapters/exif",
     "crates/adapters/thumbnail",
+    "crates/adapters/sidecar",
     "crates/presentation",
     "crates/bootstrap",
     "crates/worker",
@@ -50,6 +51,7 @@ adapters-event-transport = { path = "crates/adapters/event-transport" }
 adapters-nats      = { path = "crates/adapters/nats" }
 adapters-exif      = { path = "crates/adapters/exif" }
 adapters-thumbnail = { path = "crates/adapters/thumbnail" }
+adapters-sidecar   = { path = "crates/adapters/sidecar" }
 adapters-postgres  = { path = "crates/adapters/postgres" }
 async-nats       = "0.48"
 async-stream     = "0.3"