0.3 Package references

Add SixLabors.ImageSharp 3.1.11 (last open-source version; 4.0 requires
a paid license), MQTTnet 4.3.7.1207 (v4 matches the APIs referenced in
SPEC/PLAN), and Serilog (AspNetCore + Console + File sinks).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

IMPLEMENTATION.md

@@ -20,7 +20,7 @@ When working through this file, mark increments complete by changing `[ ]` to `[
 - `GET /health` returning `{ status: "Healthy", mqttConnected: false }` (mqttConnected hardcoded for now).
 - **DoD:** `dotnet run --project src/FrameProcessor` then `curl localhost:8080/health` returns 200 with the JSON.
 
-### [ ] 0.3 Package references
+### [x] 0.3 Package references
 - Add to `FrameProcessor.csproj`:
   - `SixLabors.ImageSharp`
   - `MQTTnet`