MCP-03: Add MCP blockstore CFs and conflict-safe put/get APIs

[aeyakovenko]

Summary

• add MCP-dedicated blockstore column families:
  • mcp_shred_data keyed by (slot, proposer_index, shred_index)
  • mcp_relay_attestation keyed by (slot, relay_index)
• wire both CFs into RocksDB descriptor/name lists
• add Blockstore APIs for MCP put/get with deterministic non-overwrite conflict reporting:
  • McpPutStatus::{Inserted, Duplicate, Conflict(McpConflictMarker)}
• include MCP CFs in purge paths and CF metrics submission
• extend tests for conflict behavior and purge coverage

Why this shape

• keeps the existing Agave shred pipeline types untouched
• keeps MCP storage isolated in dedicated CFs
• preserves replay-visible equivocation evidence without silent overwrite

Tests

• cargo test -p solana-ledger --lib test_mcp_shred_data_put_get_and_conflict -- --nocapture
• cargo test -p solana-ledger --lib test_mcp_relay_attestation_put_get_and_conflict -- --nocapture
• cargo test -p solana-ledger --lib test_purge_slots -- --nocapture
• cargo test -p solana-ledger --lib test_cf_names_and_descriptors_equal_length -- --nocapture

Fixes #2