refactor: compare a CompoundCycles on a part it names

[mraszyk]

CompoundCycles derived Ord and PartialOrd over its fields, so a comparison of two amounts was made on their real parts first and only fell back to the nominal ones for amounts whose real parts are equal.

Which part a comparison is made on matters. The real part of an amount whose use case is free under the free cost schedule is zero (cf. CompoundCycles::new), so a comparison of real parts cannot tell two such amounts apart; the nominal part is independent of the cost schedule. Under the normal cost schedule the two parts coincide, so the distinction is invisible there.

Nothing derived that ordering on purpose, and all four call sites that used it clamp a cost to a prepayment, for which the nominal part is the right one:

• CyclesAccountManager::refund_unused_execution_cycles
• CyclesAccountManager::settle_prepayment_for_unexecuted_response
• CyclesAccountManager::refund_for_response_transmission
• CanisterManager, which recomputes the same clamp to record the net cycle charge

They were correct only because the real and the nominal parts coincide under the normal cost schedule and the real parts are both zero under the free one, i.e. because the fallback to the nominal part happened to kick in. Reordering the two fields of the struct would have changed what they mean.

This PR drops the two derives and adds CompoundCycles::min_nominal, which says which part it compares, and uses it at those four call sites. It is behavior-preserving: for two amounts derived under the same cost schedule, and for the use cases those call sites deal with, min_nominal picks the same amount as the derived ordering did. The tests that compared two execution costs with assert_gt! now compare their nominal parts.

Why now

Cleanup split out of #11431, which fixes a bug of exactly this shape: the cycles prepaid for a response execution were settled against the requirement by comparing their real parts, which made the adjustment a no-op under the free cost schedule and misreported the consumed cycles metrics. Removing the derived ordering means no call site can compare on a part it did not name.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The explicit comparisons preserve existing behavior while removing ambiguous implicit ordering.

Pull request overview

Makes CompoundCycles comparisons explicit to prevent accidental ordering by the real component.

Changes:

• Removes derived ordering and adds min_nominal.
• Updates four production clamps and affected test assertions.

File summaries

File	Description
rs/types/cycles/src/compound_cycles.rs	Adds explicit nominal minimum comparison.
rs/cycles_account_manager/src/cycles_account_manager.rs	Uses nominal comparisons for refunds and settlement.
rs/execution_environment/src/canister_manager.rs	Uses nominal comparison when recording charges.
rs/execution_environment/src/execution/response/tests.rs	Compares nominal execution costs explicitly.
rs/execution_environment/tests/hypervisor.rs	Compares nominal execution costs explicitly.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Balanced

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