Add remaining statistics write coverage

[MarcLeinenDE]

Add physical SDK coverage for the last two Statistics write helpers missing from the canonical API↔SDK audit.

Covered:

• statistics/set_black_white_mode
  • uses a same-state write through the public SDK
  • verifies exact mode read-back
  • avoids unnecessary Black↔White lockout risk because real mutation semantics are already live-verified upstream
• statistics/stat_clear_common_data
  • executes the intentionally destructive traffic/history clear action
  • requires statistics.setting_response=OK
  • reads counters again and, when historical totals existed, requires a reduced aggregate total
  • never logs raw traffic-counter values

API-first check:

• both methods are already LIVE_VERIFIED, ADMIN_OK and their request semantics are normalized
• no upstream contract change is required before this physical SDK run
• any new physical mismatch will be normalized in nr2301-api before merge

Keep draft until the physical router run completes.