refactor: remove SearXNG references and rename binary to kafka

- Rename cmd/searxng-go to cmd/kafka
- Remove all SearXNG references from source comments while keeping
  "SearXNG-compatible API" in user-facing docs
- Update binary paths in README, CLAUDE.md, and Dockerfile
- Update log message to "kafka starting"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/search/merge.go

@@ -8,7 +8,7 @@ import (
 	"github.com/metamorphosis-dev/kafka/internal/contracts"
 )
 
-// MergeResponses merges multiple SearXNG-compatible JSON responses.
+// MergeResponses merges multiple compatible JSON responses.
 //
 // MVP merge semantics:
 // - results are concatenated with a simple de-dup key (engine|title|url)