Persist final downlink metadata

[sylvesterkaczmarek]

Fixes #237.

Keep the existing early metadata write for crash resilience, then rewrite the .md.pb sidecar after downlink validation has established the final completion status. The final write includes time_end, so the persisted metadata now matches the FileDownlink emitted on the bus.

The metadata writer seeks to the start and truncates before rewriting, avoiding stale protobuf bytes when the final message is shorter than the early snapshot.

Validation:

• go test ./pkg/dwn
• go test ./pkg/...
• git diff --check

The regression test decodes the written sidecar and verifies that its final status and end time match the completed downlink message.

[FarkasJoseph]

lgtm; should likely be e2e tested

[sylvesterkaczmarek]

Thanks. I checked the integration harness before expanding this. The regression in this PR already drives the real DownlinkSession event loop through completion and reads the protobuf sidecar back from disk. The Docker/F Prime integration suite currently writes the backend downlink root under /app, which is not shared with the Go test process, so a true transport-to-sidecar assertion would also require changing the integration harness/runtime configuration. I would keep that harness expansion separate from this focused persistence fix.