perf: reuse SQLite prepared statements in TPCC

[KKould]

Summary

• Change the TPCC backend traits to carry backend-specific prepared statement types through the benchmark loop.
• Make the SQLite TPCC backend prepare statements once in prepare_statements and reuse them for repeated transaction execution.
• Update TPCC benchmark docs with the new full 720s SQLite results.

TPCC Results

Run with the default TPCC duration, --num-ware 1, default --max-retry 5.

Backend	TpmC	New-Order p90	Payment p90	Order-Status p90	Delivery p90	Stock-Level p90
SQLite balanced	56788	0.001s	0.001s	0.001s	0.001s	0.001s
SQLite practical	44049	0.001s	0.001s	0.001s	0.001s	0.001s

Compared with the previous README results:

Backend	Previous TpmC	New TpmC	Change
SQLite balanced	39835	56788	+42.6%
SQLite practical	37911	44049	+16.2%

Raw logs were generated under tpcc/results/2026-06-21_09-00-53-sqlite-prepared/.

Tests

• cargo fmt --check
• cargo check -p tpcc
• cargo test -p tpcc
• cargo run -p tpcc -- --backend sqlite --sqlite-profile balanced --measure-time 1 --num-ware 1 --path target/tpcc-smoke-sqlite
• cargo run -p tpcc -- --backend dual --measure-time 1 --num-ware 1 --path target/tpcc-smoke-dual
• target/release/tpcc --backend sqlite --sqlite-profile balanced --num-ware 1 --path target/tpcc-sqlite-prepared/sqlite-balanced.sqlite
• target/release/tpcc --backend sqlite --sqlite-profile practical --num-ware 1 --path target/tpcc-sqlite-prepared/sqlite-practical.sqlite
• git diff --cached --check