Add BD_SOCKET environment variable support to override daemon socket path,
enabling parallel test isolation via t.TempDir() + t.Setenv().
Changes:
- getSocketPath() checks BD_SOCKET first, falls back to dbPath-derived path
- getSocketPathForPID() checks BD_SOCKET first (for consistency)
- Add daemon_socket_test.go with isolation pattern examples
This is a minimal tracer bullet to validate the approach before
expanding to full test isolation infrastructure.
Backward compatible: default behavior unchanged without env var set.
