MCP-08: add MCP fee-payer multiplier and cumulative slot tracking

[aeyakovenko]

Summary

• add multiplier-aware fee payer validation in svm::account_loader:
  • validate_fee_payer_with_multiplier(...)
  • validate_fee_payer_for_mcp(...) using MCP_NUM_PROPOSERS = 16
• keep existing validate_fee_payer(...) behavior unchanged by delegating with multiplier 1
• add McpFeePayerTracker in core::banking_stage::consumer to track per-slot cumulative reserved fees per payer
• add check_fee_payer_unlocked_mcp(...) path that:
  • computes MCP-scaled required fee (base_fee * NUM_PROPOSERS)
  • reserves cumulatively per slot/payer
  • enforces nonce rent minimum in reservation checks

Testing

• cargo test -p solana-svm --lib test_validate_fee_payer_for_mcp_multiplier -- --nocapture
• cargo test -p solana-core --lib test_mcp_fee_payer_tracker_prevents_overcommit -- --nocapture

Closes #6