Promote error-order helpers to op_engine public API

[jc-macdonald]

Move reusable accuracy-benchmarking helpers from test code into op_engine proper.

Motivation

Tim noted that helpers like _order_from_two_errors and _orders_from_errors in the accuracy test suite have value beyond tests — external users may want to benchmark solver accuracy against analytical solutions.

Scope

• Move error-order calculation functions into an appropriate op_engine module (e.g., op_engine.diagnostics or op_engine.accuracy)
• Add public API surface and type annotations
• Update test imports accordingly

Related

• Add accuracy test suite for solver methods #28 (accuracy test suite)

Ref: PR #18 review by @TimothyWillard