[Perf]: Eliminate audit and history entry allocations when auditor/store is disabled #90

Summary

Eliminate per mutation heap allocations from MutationAuditEntry and MutationHistoryEntry creation when the registered auditor or history store does not consume them ( NoOpAuditor/NoOpHistoryStore). Remove redundant defensive copy in CreateHistoryEntry.

Goal

Lower latency and GC pressure for the common case where diagnostics are configured but no real auditor/store is attached, without breaking existing public API contracts.

Problem

Every mutation execution unconditionally allocates MutationAuditEntry (via CreateSuccess) and MutationHistoryEntry (via CreateHistoryEntry) objects, even when the registered IMutationAuditor and IMutationHistoryStore implementations are no-ops that discard the data immediately. The factory runs first, allocates, then passes the result to a consumer that ignores it.

Additionally, CreateHistoryEntry called result.SideEffects.ToList() which allocated a new List<SideEffect> on every commit a redundant copy of an already read only collection.

IMutationAuditor (src/Abstractions/Audit/IMutationAuditor.cs):

• Add bool IsEnabled => true; default interface method
• Implementations that do not consume entries override to return false

IMutationHistoryStore (src/Abstractions/History/IMutationHistoryStore.cs):

• Add bool IsEnabled => true; default interface method
• Implementations that do not consume entries override to return false

MutationExecutionOutcomeProcessor (src/Runtime/Internal/Execution/MutationExecutionOutcomeProcessor.cs):

• Guard AuditSuccessAsync/AuditFailureAsync with if (!_auditor.IsEnabled) return;
• Guard StoreInHistoryAsync with if (!_historyStore.IsEnabled) return;

MutationAuditEntryFactory (src/Runtime/Diagnostics/MutationAuditEntryFactory.cs):

• Replace result.SideEffects.ToList() with existing IReadOnlyList instance no defensive copy

NoOpAuditor / NoOpHistoryStore (Benchmarks/Diagnostics/Support/):

• Add bool IsEnabled => false; override

Acceptance Criteria

• OutcomeProcessor skips CreateSuccess when _auditor.IsEnabled is false
• OutcomeProcessor skips CreateFailure when _auditor.IsEnabled is false
• OutcomeProcessor skips CreateHistoryEntry when _historyStore.IsEnabled is false
• CreateHistoryEntry no longer calls .ToList() on side effects
• NoOpAuditor and NoOpHistoryStore declare IsEnabled => false
• All existing unit tests pass without modification
• Benchmark regression validated:

Allocation measurements for the previous baseline were not captured, so delta focuses on latency. The 2.55 KB after for NoDiagnostics_Baseline includes other required allocations audit/history entry creation itself is zero.

Non Goals

• This issue does not change Task<T> to ValueTask<T>.
• This issue does not address the with expression allocation in FinalizeResultAsync.
• This issue does not introduce any breaking public API changes.