fix(cold-sql): pool starvation and query inefficiencies

[prestwich]

Summary

Fixes ENG-2124

• Pool configuration: Replace custom PoolOverrides with sqlx::pool::PoolOptions<Any>, giving callers the full sqlx pool configuration surface. Only force max_connections = 1 for in-memory SQLite URLs (where per-connection state isolation requires it); file-backed SQLite and PostgreSQL honor caller-provided options as-is. Adds SqlConnector::with_pool_options builder method for production tunability.
• get_logs optimization: Replace COUNT(*) + SELECT with single LIMIT query — faster in both under-limit and over-limit cases.
• get_receipt consolidation: Merge 5 sequential queries (header, receipt+tx, logs, prior gas) into 2-3 via combined JOIN with COALESCE subquery.
• drain_above batch override: Replace default N+1 implementation (3N+7 queries) with batched single-transaction approach (9 queries regardless of block count).
• Integer safety: Debug assertions on to_i64/from_i64, checked tx_type conversion throughout.

Test plan

• cargo t -p signet-cold-sql --all-features — SQLite + PG conformance passing
• cargo clippy -p signet-cold-sql --all-features --all-targets — clean
• cargo clippy -p signet-cold-sql --no-default-features --all-targets — clean
• cargo clippy --workspace --all-features --all-targets — clean
• cargo +nightly fmt — clean
• ./scripts/test-postgres.sh — PostgreSQL conformance against real database

Review Fixes

• Use from_i64 for prior_cumulative_gas instead of raw as u64 cast (bypassed debug assertion)
• Use actual tx_index from DB row instead of enumerate index in drain_above
• Read first_log_index from DB instead of recomputing from log counts in drain_above
• Add debug_assert on gas_used subtraction to detect cumulative gas corruption
• Import BTreeMap at file top instead of inline fully-qualified paths
• Remove dead r_tx_index column from combined get_receipt query
• Extract shared delete_above_in_tx helper from truncate_above/drain_above
• Add comprehensive field assertions to drain_above conformance test (transaction_index, first_log_index, gas_used, tx_hash, from, block metadata)
• Enhance get_receipts_in_block and receipt lookup conformance assertions

Reviewed and updated by Claude Code.

🤖 Generated with Claude Code