feat: auditor sets weights on a block cadence

[bitzic]

• counter-weight is now periodic (~300 blocks), not one-shot per report: a validator must keep setting weights every epoch or vTrust decays
• chain.py: get_current_block + blocks_since_weight_set (hotkey -> uid ->
  blocks_since_last_update)
• weights.py: is_weight_set_due + weight_set_interval_blocks (env AUDITOR_WEIGHT_INTERVAL_BLOCKS, default 300) + auditor_hotkey_ss58
• main.py: maybe_counter_weight re-sets from the latest CLEAN epoch's replayed scores when due; runs every --once/--loop tick, decoupled from whether a new report dropped
• default 300 blocks clears the subnet weights_rate_limit (~100)
• 25 tests: pure cadence decision + orchestration flow (no chain/wallet)