feat: clear pair config on unregister

[webmirroring]

Closes #121.

Summary

• Added a private clear_pair_config helper used by unregister_pair.
• unregister_pair now removes PairFeeBps, PairMinAmount, PairMaxAmount, and PairLiquidity after removing the pair registration.
• Preserved the existing unreg event and added a companion cfg_clr event for indexers.
• Updated tests and docs so re-registering a pair starts from default fee, bounds, and liquidity values.

Security notes

• Admin gating and idempotency are unchanged.
• Removing absent config slots remains a no-op.
• Route counters, cumulative volume, and last-route timestamp are intentionally left untouched because they are operational history, not live config.
• This prevents stale fee/bounds/liquidity config from silently reviving after unregister + re-register.

Validation

• cargo fmt -- --check
• git diff --check
• Attempted cargo test unregister -- --nocapture and cargo test pair_lifecycle -- --nocapture, but this local machine has Cargo 1.84.0 and fails before compiling because base64ct v1.8.3 requires edition2024 / Cargo 1.85+.