port: commit the Cargo.lock entry for sylpheed-export

The crate is in the workspace and builds, but its lock entry was never
committed -- `build-export` regenerates it on every fresh container, so it
only shows up as a dirty tree nobody caused.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Evuhbt8pxKJEUvwfniwYkU

Cargo.lock

@@ -4618,6 +4618,18 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "sylpheed-export"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "clap",
+ "image",
+ "serde",
+ "serde_json",
+ "sylpheed-formats",
+]
+
 [[package]]
 name = "sylpheed-formats"
 version = "0.1.0"