fix: unify weight handling, relative arb tolerance, arb fee modeling

[MatthewWilletts]

• fine_weights: unify weight handling between weight-outputting and weight-change-outputting rules — both now prepend initial weights
• optimal_n_pool_arb: switch invariant validation from absolute to relative tolerance ((post - initial) / initial >= slack), making it scale-invariant with pool TVL
• optimal_n_pool_arb: fix slack passthrough — parallelised_optimal_trade_sifter and wrapped variant were hardcoding slack=0 instead of forwarding the arg
• optimal_n_pool_arb: add arb fee adjustment functions that fold external costs into effective market prices before trade sizing, unifying the fee-aware and fee-free paths
• plot_utils: fix deprecated df.pivot(*cols) to keyword form
• price_data_fingerprint_utils: switch from CSV to parquet loading
• amalgamated_data_utils: add docstring to forward_fill_ohlcv_data

[MatthewWilletts]

reviewed